diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 98a1cb29d..49611c129 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -254,6 +254,13 @@ include(CTest)
 include(Catch)
 catch_discover_tests(tests ADD_TAGS_AS_LABELS SKIP_IS_FAILURE)
 
+option(SVS_TEST_CONCURRENT_FULL_SIZE
+    "Run the concurrent MutableVamanaIndex tests at full (stress) problem size (slow)" OFF
+)
+if (SVS_TEST_CONCURRENT_FULL_SIZE)
+    target_compile_definitions(tests PRIVATE SVS_TEST_CONCURRENT_FULL_SIZE)
+endif()
+
 #####
 ##### ThreadSanitizer targets for the concurrent index
 #####
diff --git a/tests/svs/concurrent/concurrency.cpp b/tests/svs/concurrent/concurrency.cpp
index bc5719e82..e5f4b9e9d 100644
--- a/tests/svs/concurrent/concurrency.cpp
+++ b/tests/svs/concurrent/concurrency.cpp
@@ -60,17 +60,22 @@ namespace ccg = svs::index::vamana::concurrent::graphs;
 
 namespace {
 
-// ThreadSanitizer costs roughly an order of magnitude in both time and memory, and it is
-// looking for *races*, which show up just as readily in a small index. Shrink the problem
-// rather than skipping the run.
-#if defined(__SANITIZE_THREAD__) || defined(SVS_THREAD_SANITIZER)
-constexpr size_t kInitialPoints = 4000;
-constexpr size_t kIncrementalPoints = 1000;
-constexpr size_t kNumQueries = 50;
-#else
+// Races and broken invariants show up just as readily in a small index, so the small size
+// is the default. At full size these four tests take ~10 minutes of serial ctest time on a
+// 4-vCPU CI runner, more than half of the whole suite, and run in every matrix job.
+// `SVS_TEST_CONCURRENT_FULL_SIZE` restores the larger problem for stress runs.
+//
+// ThreadSanitizer costs roughly an order of magnitude in both time and memory, so it always
+// uses the small size, even when full size is requested.
+#if defined(SVS_TEST_CONCURRENT_FULL_SIZE) && \
+    !(defined(__SANITIZE_THREAD__) || defined(SVS_THREAD_SANITIZER))
 constexpr size_t kInitialPoints = 20000;
 constexpr size_t kIncrementalPoints = 5000;
 constexpr size_t kNumQueries = 200;
+#else
+constexpr size_t kInitialPoints = 4000;
+constexpr size_t kIncrementalPoints = 1000;
+constexpr size_t kNumQueries = 50;
 #endif
 
 constexpr size_t kDim = 32;