Encoding the heap region ID into the name of the per-region mutex adds some overhead for each region. Most usage of mutexes like this provide just a static string describing the purpose of the mutex, e.g:

diff -r b588772dff7a src/hotspot/share/gc/g1/heapRegionRemSet.cpp
--- a/src/hotspot/share/gc/g1/heapRegionRemSet.cpp Thu Apr 02 15:05:24 2020 +0200
+++ b/src/hotspot/share/gc/g1/heapRegionRemSet.cpp Thu Apr 02 15:38:38 2020 +0200
@@ -373,7 +373,7 @@
                                    HeapRegion* hr)
   : _bot(bot),
     _code_roots(),
- _m(Mutex::leaf, FormatBuffer<128>("HeapRegionRemSet lock #%u", hr->hrm_index()), true, Mutex::_safepoint_check_never),
+ _m(Mutex::leaf, "HeapRegionRemSet lock", true, Mutex::_safepoint_check_never),
     _other_regions(&_m),
     _hr(hr),
     _state(Untracked)

This would remove ~15% of the remaining per-region startup execution overhead.