Performance issue in certificate handling

[shuse2]

Expected behavior

certificate resource usage should be bounded and should not affect other parts

Actual behavior

In some cases, certificate cases everything to slow down. CPU over 60%, slow block generation and execution and slow RPC response time.

Steps to reproduce

Run a network and wait for the certificate to be generated for a while (around 1.5k blocks)
However, this doesn't happen all the time

Which version(s) does this affect? (Environment, OS, etc...)

6.0.0-alpha.16