diff --git a/Tests/LibJS/test-js.cpp b/Tests/LibJS/test-js.cpp
index 74d0cd8db2b..988987dd50c 100644
--- a/Tests/LibJS/test-js.cpp
+++ b/Tests/LibJS/test-js.cpp
@@ -8,6 +8,7 @@
 #include <AK/Enumerate.h>
 #include <LibCore/Environment.h>
 #include <LibJS/Runtime/ArrayBuffer.h>
+#include <LibJS/Runtime/Date.h>
 #include <LibJS/Runtime/TypedArray.h>
 #include <LibTest/JavaScriptTestRunner.h>
 #include <LibUnicode/TimeZone.h>
@@ -109,6 +110,7 @@ TESTJS_GLOBAL_FUNCTION(set_time_zone, setTimeZone)
             return vm.throw_completion<JS::InternalError>(MUST(String::formatted("Could not set time zone: {}", result.error())));
     }
 
+    JS::clear_system_time_zone_cache();
     Unicode::clear_system_time_zone_cache();
     tzset();
 
diff --git a/Userland/Libraries/LibJS/Runtime/Date.cpp b/Userland/Libraries/LibJS/Runtime/Date.cpp
index b36ff27bbcd..e73fa826493 100644
--- a/Userland/Libraries/LibJS/Runtime/Date.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Date.cpp
@@ -411,9 +411,15 @@ Unicode::TimeZoneOffset get_named_time_zone_offset_nanoseconds(StringView time_z
     return offset.release_value();
 }
 
+static Optional<String> cached_system_time_zone_identifier;
+
 // 21.4.1.24 SystemTimeZoneIdentifier ( ), https://tc39.es/ecma262/#sec-systemtimezoneidentifier
 String system_time_zone_identifier()
 {
+    // OPTIMIZATION: We cache the system time zone to avoid the expensive lookups below.
+    if (cached_system_time_zone_identifier.has_value())
+        return *cached_system_time_zone_identifier;
+
     // 1. If the implementation only supports the UTC time zone, return "UTC".
 
     // 2. Let systemTimeZoneString be the String representing the host environment's current time zone, either a primary
@@ -429,7 +435,13 @@ String system_time_zone_identifier()
     }
 
     // 3. Return systemTimeZoneString.
-    return system_time_zone_string;
+    cached_system_time_zone_identifier = move(system_time_zone_string);
+    return *cached_system_time_zone_identifier;
+}
+
+void clear_system_time_zone_cache()
+{
+    cached_system_time_zone_identifier.clear();
 }
 
 // 21.4.1.25 LocalTime ( t ), https://tc39.es/ecma262/#sec-localtime
diff --git a/Userland/Libraries/LibJS/Runtime/Date.h b/Userland/Libraries/LibJS/Runtime/Date.h
index 3a2e10e1c6c..aca9a0c3015 100644
--- a/Userland/Libraries/LibJS/Runtime/Date.h
+++ b/Userland/Libraries/LibJS/Runtime/Date.h
@@ -77,6 +77,7 @@ Crypto::SignedBigInteger get_utc_epoch_nanoseconds(i32 year, u8 month, u8 day, u
 Vector<Crypto::SignedBigInteger> get_named_time_zone_epoch_nanoseconds(StringView time_zone_identifier, i32 year, u8 month, u8 day, u8 hour, u8 minute, u8 second, u16 millisecond, u16 microsecond, u16 nanosecond);
 Unicode::TimeZoneOffset get_named_time_zone_offset_nanoseconds(StringView time_zone_identifier, Crypto::SignedBigInteger const& epoch_nanoseconds);
 String system_time_zone_identifier();
+void clear_system_time_zone_cache();
 double local_time(double time);
 double utc_time(double time);
 double make_time(double hour, double min, double sec, double ms);
diff --git a/Userland/Services/WebContent/ConnectionFromClient.cpp b/Userland/Services/WebContent/ConnectionFromClient.cpp
index cdba8d60f39..3e8faa38182 100644
--- a/Userland/Services/WebContent/ConnectionFromClient.cpp
+++ b/Userland/Services/WebContent/ConnectionFromClient.cpp
@@ -16,6 +16,7 @@
 #include <LibGfx/SystemTheme.h>
 #include <LibJS/Heap/Heap.h>
 #include <LibJS/Runtime/ConsoleObject.h>
+#include <LibJS/Runtime/Date.h>
 #include <LibUnicode/TimeZone.h>
 #include <LibWeb/ARIA/RoleType.h>
 #include <LibWeb/Bindings/MainThreadVM.h>
@@ -1194,6 +1195,7 @@ void ConnectionFromClient::enable_inspector_prototype(u64)
 
 void ConnectionFromClient::system_time_zone_changed()
 {
+    JS::clear_system_time_zone_cache();
     Unicode::clear_system_time_zone_cache();
 }