diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index df4e52e5d..69d7ba380 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1090,7 +1090,9 @@ void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
 void EvalState::resetFileCache()
 {
     fileEvalCache.clear();
+    fileEvalCache.rehash(0);
     fileParseCache.clear();
+    fileParseCache.rehash(0);
     inputCache->clear();
 }
 
@@ -2375,10 +2377,9 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
     if (nix::isDerivation(path.path.abs()))
         error<EvalError>("file names are not allowed to end in '%1%'", drvExtension).debugThrow();
 
-    auto dstPathCached = get(*srcToStore.lock(), path);
-
-    auto dstPath = dstPathCached ? *dstPathCached : [&]() {
-        auto dstPath = fetchToStore(
+    std::optional<StorePath> dstPath;
+    if (!srcToStore.cvisit(path, [&dstPath](const auto & kv) { dstPath.emplace(kv.second); })) {
+        dstPath.emplace(fetchToStore(
             fetchSettings,
             *store,
             path.resolveSymlinks(SymlinkResolution::Ancestors),
@@ -2386,15 +2387,14 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
             path.baseName(),
             ContentAddressMethod::Raw::NixArchive,
             nullptr,
-            repair);
-        allowPath(dstPath);
-        srcToStore.lock()->try_emplace(path, dstPath);
-        printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
-        return dstPath;
-    }();
+            repair));
+        allowPath(*dstPath);
+        srcToStore.try_emplace(path, *dstPath);
+        printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(*dstPath));
+    }
 
-    context.insert(NixStringContextElem::Opaque{.path = dstPath});
-    return dstPath;
+    context.insert(NixStringContextElem::Opaque{.path = *dstPath});
+    return *dstPath;
 }
 
 SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx)
diff --git a/src/libexpr/include/nix/expr/eval.hh b/src/libexpr/include/nix/expr/eval.hh
index 5015a009b..75ed12664 100644
--- a/src/libexpr/include/nix/expr/eval.hh
+++ b/src/libexpr/include/nix/expr/eval.hh
@@ -20,6 +20,8 @@
 // For `NIX_USE_BOEHMGC`, and if that's set, `GC_THREADS`
 #include "nix/expr/config.hh"
 
+#include <boost/unordered/concurrent_flat_map.hpp>
+#include <boost/unordered/unordered_flat_map.hpp>
 #include <map>
 #include <optional>
 #include <functional>
@@ -162,7 +164,7 @@ typedef std::
     map<std::string, Value *, std::less<std::string>, traceable_allocator<std::pair<const std::string, Value *>>>
         ValMap;
 
-typedef std::unordered_map<PosIdx, DocComment> DocCommentMap;
+typedef boost::unordered_flat_map<PosIdx, DocComment, std::hash<PosIdx>> DocCommentMap;
 
 struct Env
 {
@@ -395,7 +397,7 @@ public:
     bool inDebugger = false;
     int trylevel;
     std::list<DebugTrace> debugTraces;
-    std::map<const Expr *, const std::shared_ptr<const StaticEnv>> exprEnvs;
+    boost::unordered_flat_map<const Expr *, const std::shared_ptr<const StaticEnv>> exprEnvs;
 
     const std::shared_ptr<const StaticEnv> getStaticEnv(const Expr & expr) const
     {
@@ -438,12 +440,12 @@ private:
 
     /* Cache for calls to addToStore(); maps source paths to the store
        paths. */
-    Sync<std::unordered_map<SourcePath, StorePath>> srcToStore;
+    boost::concurrent_flat_map<SourcePath, StorePath, std::hash<SourcePath>> srcToStore;
 
     /**
      * A cache from path names to parse trees.
      */
-    typedef std::unordered_map<
+    typedef boost::unordered_flat_map<
         SourcePath,
         Expr *,
         std::hash<SourcePath>,
@@ -455,7 +457,7 @@ private:
     /**
      * A cache from path names to values.
      */
-    typedef std::unordered_map<
+    typedef boost::unordered_flat_map<
         SourcePath,
         Value,
         std::hash<SourcePath>,
@@ -468,11 +470,11 @@ private:
      * Associate source positions of certain AST nodes with their preceding doc comment, if they have one.
      * Grouped by file.
      */
-    std::unordered_map<SourcePath, DocCommentMap> positionToDocComment;
+    boost::unordered_flat_map<SourcePath, DocCommentMap, std::hash<SourcePath>> positionToDocComment;
 
     LookupPath lookupPath;
 
-    std::map<std::string, std::optional<SourcePath>> lookupPathResolved;
+    boost::unordered_flat_map<std::string, std::optional<SourcePath>> lookupPathResolved;
 
     /**
      * Cache used by prim_match().
@@ -746,7 +748,7 @@ public:
     /**
      * Internal primops not exposed to the user.
      */
-    std::unordered_map<
+    boost::unordered_flat_map<
         std::string,
         Value *,
         std::hash<std::string>,
@@ -1017,10 +1019,10 @@ private:
 
     bool countCalls;
 
-    typedef std::map<std::string, size_t> PrimOpCalls;
+    typedef boost::unordered_flat_map<std::string, size_t> PrimOpCalls;
     PrimOpCalls primOpCalls;
 
-    typedef std::map<ExprLambda *, size_t> FunctionCalls;
+    typedef boost::unordered_flat_map<ExprLambda *, size_t> FunctionCalls;
     FunctionCalls functionCalls;
 
     /** Evaluation/call profiler. */
@@ -1028,7 +1030,7 @@ private:
 
     void incrFunctionCall(ExprLambda * fun);
 
-    typedef std::map<PosIdx, size_t> AttrSelects;
+    typedef boost::unordered_flat_map<PosIdx, size_t, std::hash<PosIdx>> AttrSelects;
     AttrSelects attrSelects;
 
     friend struct ExprOpUpdate;
diff --git a/src/libexpr/include/nix/expr/parser-state.hh b/src/libexpr/include/nix/expr/parser-state.hh
index 836cc9861..e689678de 100644
--- a/src/libexpr/include/nix/expr/parser-state.hh
+++ b/src/libexpr/include/nix/expr/parser-state.hh
@@ -71,7 +71,7 @@ struct LexerState
     /**
      * @brief Maps some positions to a DocComment, where the comment is relevant to the location.
      */
-    std::unordered_map<PosIdx, DocComment> & positionToDocComment;
+    DocCommentMap & positionToDocComment;
 
     PosTable & positions;
     PosTable::Origin origin;
diff --git a/src/libexpr/include/nix/expr/value.hh b/src/libexpr/include/nix/expr/value.hh
index 82db1a775..d3aeac157 100644
--- a/src/libexpr/include/nix/expr/value.hh
+++ b/src/libexpr/include/nix/expr/value.hh
@@ -12,6 +12,7 @@
 #include "nix/expr/print-options.hh"
 #include "nix/util/checked-arithmetic.hh"
 
+#include <boost/unordered/unordered_flat_map_fwd.hpp>
 #include <nlohmann/json_fwd.hpp>
 
 namespace nix {
@@ -1166,7 +1167,7 @@ void Value::mkBlackhole()
 }
 
 typedef std::vector<Value *, traceable_allocator<Value *>> ValueVector;
-typedef std::unordered_map<
+typedef boost::unordered_flat_map<
     Symbol,
     Value *,
     std::hash<Symbol>,
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 35fe929d9..89da001ef 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -57,7 +57,7 @@
 
 namespace nix {
 
-typedef std::unordered_map<PosIdx, DocComment> DocCommentMap;
+typedef boost::unordered_flat_map<PosIdx, DocComment, std::hash<PosIdx>> DocCommentMap;
 
 Expr * parseExprFromBuf(
     char * text,
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 9ba417c32..c107c6bc2 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -18,6 +18,8 @@
 #include "nix/util/sort.hh"
 
 #include <boost/container/small_vector.hpp>
+#include <boost/unordered/concurrent_flat_map.hpp>
+#include <boost/unordered/unordered_flat_map.hpp>
 #include <nlohmann/json.hpp>
 
 #include <sys/types.h>
@@ -1750,7 +1752,7 @@ static void derivationStrictInternal(EvalState & state, std::string_view drvName
        read them later. */
     {
         auto h = hashDerivationModulo(*state.store, drv, false);
-        drvHashes.lock()->insert_or_assign(drvPath, h);
+        drvHashes.insert_or_assign(drvPath, std::move(h));
     }
 
     auto result = state.buildBindings(1 + drv.outputs.size());
@@ -4027,7 +4029,7 @@ static void prim_groupBy(EvalState & state, const PosIdx pos, Value ** args, Val
         auto name = state.forceStringNoCtx(
             res, pos, "while evaluating the return value of the grouping function passed to builtins.groupBy");
         auto sym = state.symbols.create(name);
-        auto vector = attrs.try_emplace(sym, ValueVector()).first;
+        auto vector = attrs.try_emplace<ValueVector>(sym, {}).first;
         vector->second.push_back(vElem);
     }
 
@@ -4562,27 +4564,19 @@ static RegisterPrimOp primop_convertHash({
 
 struct RegexCache
 {
-    struct State
-    {
-        std::unordered_map<std::string, std::regex, StringViewHash, std::equal_to<>> cache;
-    };
-
-    Sync<State> state_;
+    boost::concurrent_flat_map<std::string, std::regex, StringViewHash, std::equal_to<>> cache;
 
     std::regex get(std::string_view re)
     {
-        auto state(state_.lock());
-        auto it = state->cache.find(re);
-        if (it != state->cache.end())
-            return it->second;
+        std::regex regex;
         /* No std::regex constructor overload from std::string_view, but can be constructed
            from a pointer + size or an iterator range. */
-        return state->cache
-            .emplace(
-                std::piecewise_construct,
-                std::forward_as_tuple(re),
-                std::forward_as_tuple(/*s=*/re.data(), /*count=*/re.size(), std::regex::extended))
-            .first->second;
+        cache.try_emplace_and_cvisit(re,
+            /*s=*/re.data(), /*count=*/re.size(), std::regex::extended,
+            [&regex](const auto & kv) { regex = kv.second; },
+            [&regex](const auto & kv) { regex = kv.second; }
+        );
+        return regex;
     }
 };
 
@@ -4826,7 +4820,7 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value ** ar
         from.emplace_back(state.forceString(
             *elem, pos, "while evaluating one of the strings to replace passed to builtins.replaceStrings"));
 
-    std::unordered_map<size_t, std::string_view> cache;
+    boost::unordered_flat_map<size_t, std::string_view> cache;
     auto to = args[1]->listView();
 
     NixStringContext context;