ES performance improvement after restarting ES instance?

I encountered a strange phenomenon where the Elasticsearch instance performed better after restarting it during performance testing with esrally. i don't know why?
esrally :v2.6.0 (official http_logs track)
es : v7.17.9 (only one es instance)
x86_64 ,16 cpu, 64G mem( jvm use 31G)
step:
1.

esrally race --track="http_logs" --target-hosts="$server_hosts" --pipeline=benchmark-only --kill-running-processes
  1. because I am only interested in range queries. When the target-throughput of range queries is set to 230 with 16 clients, the CPU load is at 100% capacity.
esrally race --track=http_logs --target-hosts="$server_hosts" --pipeline=benchmark-only --kill-running-processes --offline --include-tasks="range"
  1. restart es instance
    4.Running the following command with the same target-throughput and clients,the CPU load is only at 50% capacity, but the performance results output by esrally are consistent with the results obtained at full load.
esrally race --track=http_logs --target-hosts="$server_hosts" --pipeline=benchmark-only --kill-running-processes --offline --include-tasks="range"