Most of my experience with this actually comes from RoR. I worked on one so where rendering slowed everything down to a grind, but more often than not it was making Yu noptimized queries, too many queries, inefficient calls to other services. I used to work as a consultant, so this was true for quite a few apps. However, I also usually was the person most interested in relational databases and my view might have selection bias.

I have a similar experience. Also, for the longest time Rails was also incredibly slow at generating JSON, of all things. JBuilder [1] was a few orders of magnitude slower than using .to_json directly or other libraries.

[1] https://github.com/rails/jbuilder, maintained by DHH and the Rails team. AFAIR, the "official" JSON serialization DSL.