Do non-native English speakers appreciate it when I carefully correct their English in peer reviews? As a native English speaker, when I review papers I sometimes carefully go through the English and list my proposed changes, which adds a fair amount of time to the review process.

The most extreme case of this was for a good paper I reviewed for a top computer-science journal, but with fairly poor English.  I went through everything in detail, marking proposed changes on a hard copy, then sent back a scan with my review.  This kind of review typically adds hours, possibly even days.

It makes me wonder if it's at all appreciated.  Perhaps the authors groan when they see such reviews, thinking there's a lot of tedious legwork to do.  Or, as I hope, they actually learn something meaningful from this effort.  I don't really know.

Question: Do non-native English speakers appreciate it when I carefully correct their English in peer reviews?
I think it is a good idea.  Yes, even in mathematics.  And even papers by native English speakers.

Sometimes, if the English has many errors, I just note that in my review, and suggest the author have the paper corrected by an English speaker before re-submitting.

Journals should have their own people to correct English after papers are accepted.  But who knows what happens with modern free-access no-fee journals who cannot afford such luxuries?