huge difference of performance gain when memfile is configured with persist false

I've executed two tests, multi threading + memfile with persist false (there is no writing to the file). And results are surprising, revealing possible inefficiency.

results:

• multi threading v6 memfile persist true: 36k leases/s
• multi threading v6 memfile persist false: 47k leases/s
  • gain: 30%
• multi threading v4 memfile persist true: 20k leases/s
• multi threading v4 memfile persist false: 37k leases/s
  • gain: 85%

Please investigate if there is an inefficiency in a way Kea saves v4 leases to the file.