[2021] Anybody else getting massive lag spikes after the LEM update?

I can't say for myself as I don't play this game any more, but I haven't seen other such reports here, no.

Is this in a new galaxy started under 3.1.1 ?

When you disabled mods did you then test under a new unmodded galaxy? Because disabling mods does not remove effects they've already had on the save game file.  

If you revert to a 3.0 version in the Steam betas tab, and again test with a new galaxy unmodded, do you still get this problem?  

I see. Probably time to take this up with QA and the devs in the Bug Reports forum then!

Sorry about that.