One thing I didn't see discussed in the article was YJIT's memory usage relative to CRuby, the baseline non-JIT version of Ruby. It is certainly possible I missed it; that's been known to happen!
Anyway, the news there is very good. We can see detailed information here:
https://speed.yjit.org/memory_timeline.html#railsbench
Currently Railsbench consumes a peak of ~95MB with CRuby, and a peak of ~110MB with YJIT. So, YJIT delivers 70% more performance while consuming 16% more RAM here. That is a tradeoff I think most people would gladly accept in most scenarios. =)
Real-world speedups will be less, since a "real" web application spends much of its time waiting for the database and other external resources. As the article notes, Shopify's real-world observed storefront perf gain is 27.2%.
YJIT is a success and its future is even brighter.