Eval performance regression

[Simn]

I noticed some eval benchmarks that suffered performance regressions on ~~May 23rd~~ March 23rd:

This is quite surprising because compilation time in general (greatly) improved on that day, which we attribute to the change from OCaml 4 to 5. Eval must be doing something specific that OCaml 5 doesn't like, so some good old-fashioned profiling is in order here. (sad Windows noises...)