this is the winning combination using two threads on my laptop.

3e744493 · Pedro Gonnet · c0a5abd4 · 3e744493
Commit 3e744493 authored Jul 10, 2017 by Pedro Gonnet
--- a/src/space.h
+++ b/src/space.h
@@ -44,7 +44,7 @@ struct cell;
 #define space_splitsize_default 400
 #define space_maxsize_default 8000000
 #define space_subsize_pair_default 64000000
-#define space_subsize_self_default 8000
+#define space_subsize_self_default 16000
 #define space_maxcount_default 10000
 #define space_max_top_level_cells_default 12
 #define space_stretch 1.10f