diff --git a/src/ArenaPool.zig b/src/ArenaPool.zig
index 32257e30..7f1d38dc 100644
--- a/src/ArenaPool.zig
+++ b/src/ArenaPool.zig
@@ -29,6 +29,7 @@ free_list_len: u16 = 0,
 free_list: ?*Entry = null,
 free_list_max: u16,
 entry_pool: std.heap.MemoryPool(Entry),
+mutex: std.Thread.Mutex = .{},
 
 const Entry = struct {
     next: ?*Entry,
@@ -54,6 +55,9 @@ pub fn deinit(self: *ArenaPool) void {
 }
 
 pub fn acquire(self: *ArenaPool) !Allocator {
+    self.mutex.lock();
+    defer self.mutex.unlock();
+
     if (self.free_list) |entry| {
         self.free_list = entry.next;
         self.free_list_len -= 1;
@@ -73,6 +77,12 @@ pub fn release(self: *ArenaPool, allocator: Allocator) void {
     const arena: *std.heap.ArenaAllocator = @ptrCast(@alignCast(allocator.ptr));
     const entry: *Entry = @fieldParentPtr("arena", arena);
 
+    // Reset the arena before acquiring the lock to minimize lock hold time
+    _ = arena.reset(.{ .retain_with_limit = self.retain_bytes });
+
+    self.mutex.lock();
+    defer self.mutex.unlock();
+
     const free_list_len = self.free_list_len;
     if (free_list_len == self.free_list_max) {
         arena.deinit();
@@ -80,7 +90,6 @@ pub fn release(self: *ArenaPool, allocator: Allocator) void {
         return;
     }
 
-    _ = arena.reset(.{ .retain_with_limit = self.retain_bytes });
     entry.next = self.free_list;
     self.free_list_len = free_list_len + 1;
     self.free_list = entry;
diff --git a/src/browser/Robots.zig b/src/browser/Robots.zig
index 5c0428fa..0318ef30 100644
--- a/src/browser/Robots.zig
+++ b/src/browser/Robots.zig
@@ -111,12 +111,16 @@ pub const RobotStore = struct {
 
     allocator: std.mem.Allocator,
     map: RobotsMap,
+    mutex: std.Thread.Mutex = .{},
 
     pub fn init(allocator: std.mem.Allocator) RobotStore {
         return .{ .allocator = allocator, .map = .empty };
     }
 
     pub fn deinit(self: *RobotStore) void {
+        self.mutex.lock();
+        defer self.mutex.unlock();
+
         var iter = self.map.iterator();
 
         while (iter.next()) |entry| {
@@ -132,6 +136,9 @@ pub const RobotStore = struct {
     }
 
     pub fn get(self: *RobotStore, url: []const u8) ?RobotsEntry {
+        self.mutex.lock();
+        defer self.mutex.unlock();
+
         return self.map.get(url);
     }
 
@@ -140,11 +147,17 @@ pub const RobotStore = struct {
     }
 
     pub fn put(self: *RobotStore, url: []const u8, robots: Robots) !void {
+        self.mutex.lock();
+        defer self.mutex.unlock();
+
         const duped = try self.allocator.dupe(u8, url);
         try self.map.put(self.allocator, duped, .{ .present = robots });
     }
 
     pub fn putAbsent(self: *RobotStore, url: []const u8) !void {
+        self.mutex.lock();
+        defer self.mutex.unlock();
+
         const duped = try self.allocator.dupe(u8, url);
         try self.map.put(self.allocator, duped, .absent);
     }
diff --git a/src/browser/js/Env.zig b/src/browser/js/Env.zig
index 8b64ad1f..ef0cc1f6 100644
--- a/src/browser/js/Env.zig
+++ b/src/browser/js/Env.zig
@@ -39,6 +39,14 @@ const JsApis = bridge.JsApis;
 const Allocator = std.mem.Allocator;
 const IS_DEBUG = builtin.mode == .Debug;
 
+fn initClassIds() void {
+    inline for (JsApis, 0..) |JsApi, i| {
+        JsApi.Meta.class_id = i;
+    }
+}
+
+var class_id_once = std.once(initClassIds);
+
 // The Env maps to a V8 isolate, which represents a isolated sandbox for
 // executing JavaScript. The Env is where we'll define our V8 <-> Zig bindings,
 // and it's where we'll start ExecutionWorlds, which actually execute JavaScript.
@@ -90,6 +98,9 @@ pub fn init(app: *App, opts: InitOpts) !Env {
         }
     }
 
+    // Initialize class IDs once before any V8 work
+    class_id_once.call();
+
     const allocator = app.allocator;
     const snapshot = &app.snapshot;
 
@@ -132,8 +143,7 @@ pub fn init(app: *App, opts: InitOpts) !Env {
         temp_scope.init(isolate);
         defer temp_scope.deinit();
 
-        inline for (JsApis, 0..) |JsApi, i| {
-            JsApi.Meta.class_id = i;
+        inline for (JsApis, 0..) |_, i| {
             const data = v8.v8__Isolate__GetDataFromSnapshotOnce(isolate_handle, snapshot.data_start + i);
             const function_handle: *const v8.FunctionTemplate = @ptrCast(data);
             // Make function template eternal