Does MW not store diffs as diffs (I'd think it would for storage efficiency)? That shouldn't really require much computation. Did diffs take 30s+ to render 15-20 years ago?
For what it's worth my kiwix copy of Wikipedia has a ~5ms response time for an uncached article according to Firefox. If I hit a single URL with wrk (so some caching at least with disks. Don't know what else kiwix might do) at concurrency 8, it does 13k rps on my n305 with a 500 us average response time. That's over 20Gbit/s, so basically impossible to actually saturate. If I load test from another computer it uses ~0.2 cores to max out 1Gbit/s. Different code bases and presumably kiwix is a bit more static, but at least provides a little context to compare with for orders of magnitude. A 3 OOM difference seems pretty extreme.
Incidentally, local copies of things are pretty great. It really makes you notice how slow the web is when links open in like 1 frame.