I have compiled all versions of mailutils, starting from 3.11 and up to 3.15, on my two different PCs (Linux mint 20.3 and Gentoo x86_64): when processing a simple rule (with maildir) for about 1350 emails starting from version 3.13, the sieve process spends about 10 seconds (in version 3.12, about 0.5 seconds), with 1 CPU core loaded at 100%.

My command:
sieve ~/.imail/sieve-to -f ~/.imail/sample/Sent

My rule:
require "fileinto";
if header :contains "to" ["sample@gmx.de"]
  {
    fileinto "~/.imail/sample/test";
  }