Nimbus t8n performance issues

[winsvega]

Nimbus t8n just got super slow recently (~1/2 weeks I guess)

The bug somewhere in cycles. Especially noticeable in stQuadraticComplexityTest

[kdeme]

Possible suspect would be this PR: #1552

CI appears to also have slowed down on nimvm2 tasks since then.

[AdamSpitz]

Sorry about that. I'm trying to get my stateless-mode stuff merged in before I leave; it's very possible that there'll be performance regressions. I can look into it, but I'm running out of time and I'm not super-productive lately (which is part of the reason why I'm leaving). I asked Zahary whether I should merge this stuff in even though it's not really ready yet, and he said yes; maybe I shouldn't? I do have some thoughts on where I might have introduced performance problems. I'll take a look.