The only KV changes between RC3 and RC4 are:
Commit: 2a368c39481ff4d42c6f755bd7d185b9a57554ca in build: 6.5.0-4959
MB-37294: Avoid starvation of low-pri VBs in Flusher::flushVB()
Commit: dee7c17e96ca42fd2fd186a3c2e690c9d7231f68 in build: 6.5.0-4957
MB-37150: Register cursor before backfilling after cursor dropping
I don't believe either would be responsible for the significant performance drop we are seeing.