They were not nearly this bad in 1.16.3, where they would only affect my fps by a fraction. (from 45-50 down to 30ish)
In the 2.4.3 version of the pack, they completely obliterate my fps down to single digits (2-5 fps)

Confirmed with another player on a beast machine, managed to cut his fps down by a majority of his frames too.

I tried mining manually in the cobble gen, with the machines turned off, it had the same effect. Issue is not with Astromine but in the lava+water interaction.

EDIT: Just to clarify, mining manually in a tunnel straight forward does not produce the frame drops, but it is specifically the creation of cobblestone with lava and water that causes severe frame drops.