When you GC.start() you are probably forcing a round of garbage collection. I assume it is slow to run GC.start(), but after gives you some runtime advantages,

When you GC.start() you are probably forcing a round of garbage collection. I assume it is slow to run GC.start(), but after gives you some runtime advantages, but only for a bit (until you call it again), but then the heap fills up again. You can tune the RUBY_GC_HEAP_GROWTH_FACTOR in GC.stat after initializing and load testing a few times to avoid having to call GC.start() and manually force garbage collections by allocating enough space for the system to run its own processes, while minimizing initializing unutilized memory or having too small of a heap size, which will trigger too many garbage collection runs, both requiring expensively slow kernel system calls.