diff --git a/include/mimalloc/types.h b/include/mimalloc/types.h
index 085879bd..61681138 100644
--- a/include/mimalloc/types.h
+++ b/include/mimalloc/types.h
@@ -321,8 +321,8 @@ typedef struct mi_page_s {
 
 // The max object size are checked to not waste more than 12.5% internally over the page sizes.
 // (Except for large pages since huge objects are allocated in 4MiB chunks)
-#define MI_SMALL_MAX_OBJ_SIZE             ((MI_SMALL_PAGE_SIZE-MI_PAGE_INFO_SIZE)/6)   // < 11 KiB
-#define MI_MEDIUM_MAX_OBJ_SIZE            ((MI_MEDIUM_PAGE_SIZE-MI_PAGE_INFO_SIZE)/4)  // < 128 KiB
+#define MI_SMALL_MAX_OBJ_SIZE             ((MI_SMALL_PAGE_SIZE-MI_PAGE_INFO_SIZE)/8)   // < 11 KiB
+#define MI_MEDIUM_MAX_OBJ_SIZE            ((MI_MEDIUM_PAGE_SIZE-MI_PAGE_INFO_SIZE)/8)  // < 128 KiB
 #define MI_LARGE_MAX_OBJ_SIZE             ((MI_LARGE_PAGE_SIZE-MI_PAGE_INFO_SIZE)/4)   // < 1 MiB
 #define MI_LARGE_MAX_OBJ_WSIZE            (MI_LARGE_MAX_OBJ_SIZE/MI_SIZE_SIZE)
 
diff --git a/src/arena.c b/src/arena.c
index 044d3f39..e0044392 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -313,7 +313,7 @@ static bool mi_arena_reserve(size_t req_size, bool allow_large, mi_arena_id_t re
 
   if (arena_count >= 1 && arena_count <= 128) {
     // scale up the arena sizes exponentially every 4 entries
-    const size_t multiplier = (size_t)1 << _mi_clamp(arena_count/4, 0, 16);
+    const size_t multiplier = (size_t)1 << _mi_clamp(arena_count/2, 0, 16);
     size_t reserve = 0;
     if (!mi_mul_overflow(multiplier, arena_reserve, &reserve)) {
       arena_reserve = reserve;
diff --git a/src/bitmap.c b/src/bitmap.c
index c64f227b..a04762af 100644
--- a/src/bitmap.c
+++ b/src/bitmap.c
@@ -352,7 +352,7 @@ static inline bool mi_bchunk_clearN(mi_bchunk_t* chunk, size_t cidx, size_t n, b
   if (n==1) return mi_bchunk_clear(chunk, cidx, pmaybe_all_clear);
   if (n==MI_BFIELD_BITS) return mi_bchunk_clearX(chunk, cidx, pmaybe_all_clear);
   if (n <MI_BFIELD_BITS) return mi_bchunk_clearNX(chunk, cidx, n, pmaybe_all_clear);
-  return mi_bchunk_xsetN_(MI_BIT_CLEAR, chunk, cidx, n, NULL, pmaybe_all_clear); 
+  return mi_bchunk_xsetN_(MI_BIT_CLEAR, chunk, cidx, n, NULL, pmaybe_all_clear);
 }
 
 
@@ -596,7 +596,7 @@ static inline bool mi_bchunk_try_find_and_clear_1(mi_bchunk_t* chunk, size_t n,
   return mi_bchunk_try_find_and_clear(chunk, pidx);
 }
 
-#if !MI_OPT_SIMD
+#if !(MI_OPT_SIMD && defined(__AVX2__) && (MI_BCHUNK_BITS==512))
 static inline bool mi_bchunk_try_find_and_clear8_at(mi_bchunk_t* chunk, size_t chunk_idx, size_t* pidx, bool allow_all_set) {
   const mi_bfield_t b = mi_atomic_load_relaxed(&chunk->bfields[chunk_idx]);
   if (!allow_all_set && (~b == 0)) return false;
@@ -1277,18 +1277,18 @@ bool _mi_bitmap_forall_setc_ranges(mi_bitmap_t* bitmap, mi_forall_set_fun_t* vis
         size_t rngcount = 0;
         #endif
         size_t bidx;
-        while (mi_bfield_find_least_bit(b, &bidx)) {          
+        while (mi_bfield_find_least_bit(b, &bidx)) {
           const size_t rng = mi_ctz(~(b>>bidx)); // all the set bits from bidx
           #if MI_DEBUG > 1
           rngcount += rng;
-          #endif  
+          #endif
           mi_assert_internal(rng>=1 && rng<=MI_BFIELD_BITS);
           const size_t idx = base_idx + bidx;
           mi_assert_internal((idx % MI_BFIELD_BITS) + rng <= MI_BFIELD_BITS);
           mi_assert_internal((idx / MI_BCHUNK_BITS) < mi_bitmap_chunk_count(bitmap));
           if (!visit(idx, rng, arena, arg)) return false;
           // clear rng bits in b
-          b = b & ~mi_bfield_mask(rng, bidx);          
+          b = b & ~mi_bfield_mask(rng, bidx);
         }
         mi_assert_internal(rngcount == bpopcount);
       }
diff --git a/src/heap.c b/src/heap.c
index 1c2b017b..a24b8356 100644
--- a/src/heap.c
+++ b/src/heap.c
@@ -166,8 +166,12 @@ void mi_collect(bool force) mi_attr_noexcept {
 ----------------------------------------------------------- */
 
 mi_heap_t* mi_heap_get_default(void) {
-  mi_thread_init();
-  return mi_prim_get_default_heap();
+  mi_heap_t* heap = mi_prim_get_default_heap();
+  if mi_unlikely(!mi_heap_is_initialized(heap)) {
+    mi_thread_init();
+    heap = mi_prim_get_default_heap();
+  }
+  return heap;
 }
 
 static bool mi_heap_is_default(const mi_heap_t* heap) {
diff --git a/src/init.c b/src/init.c
index 4465d603..241a3826 100644
--- a/src/init.c
+++ b/src/init.c
@@ -157,7 +157,7 @@ mi_decl_cache_align mi_heap_t _mi_heap_main = {
   MI_BIN_FULL, 0,   // page retired min/max
   NULL,             // next heap
   MI_MEMID_STATIC,  // memid
-  0, 
+  0,
   2,                // full page retain
   true,             // allow page reclaim
   true,             // allow page abandon
@@ -289,7 +289,7 @@ mi_decl_noinline mi_tld_t* _mi_tld(void) {
   }
   if (mi_tld==NULL) {
     mi_tld = mi_tld_alloc();
-  }  
+  }
   return mi_tld;
 }
 
@@ -361,11 +361,11 @@ static bool _mi_thread_heap_init(void) {
     //mi_assert_internal(_mi_heap_default->tld->heap_backing == mi_prim_get_default_heap());
   }
   else {
-    // allocates tld data 
-    // note: we cannot access thread-locals yet as that can cause (recursive) allocation 
+    // allocates tld data
+    // note: we cannot access thread-locals yet as that can cause (recursive) allocation
     // (on macOS <= 14 for example where the loader allocates thread-local data on demand).
-    mi_tld_t* tld = mi_tld_alloc();  
-    
+    mi_tld_t* tld = mi_tld_alloc();
+
     // allocate and initialize the heap
     mi_heap_t* heap = _mi_heap_create(0 /* default tag */, false /* allow destroy? */, _mi_arena_id_none(), tld);
 
diff --git a/src/page.c b/src/page.c
index 53773aae..0e3e9bb5 100644
--- a/src/page.c
+++ b/src/page.c
@@ -870,7 +870,7 @@ void* _mi_malloc_generic(mi_heap_t* heap, size_t size, bool zero, size_t huge_al
     if mi_unlikely(!mi_heap_is_initialized(heap)) { return NULL; }
   }
   mi_assert_internal(mi_heap_is_initialized(heap));
-  
+
   // collect every N generic mallocs
   if (heap->generic_count++ > 10000) {
     heap->generic_count = 0;