Diffs load slowly in Edge

When showing a diff on the PR page in Edge, the content is displayed as black text on a gray background for 1-2 seconds depending on the size of the diff.

Edge developer tools shows that all that time is spent in scripting and styling during this. Two separate users both also noticed massive memory being used by Edge, but none if it is on the heap (it's something happening outside of JS-land and in the Edge process itself). The heap is relatively stable at 8-9MB.

It may be caused by recent LESS changes on that page because it occurred suddenly starting Feb 24th, after merging feature discovery code and some other minor changes.

It might instead be related to an Edge auto-update happening around the same time (but I don't know that one did happen, just saying it's possible).

I find it unlikely that any JS changes caused this since we didn't make any that should be related afaict.

We should investigate by running Edge against an older version of BBS to see if it's reproducible there. That will let us know if the problem is in the code we just wrote or not.