That moment when you build your app around a clients development API only to find out their production API responds about 8x slower... I'm sorry, it's not my fault your API server takes 4000 ms for a SIMPLE response. My app isn't the problem. Fix your shit.

I am in a similar position. Our shit is built with classic ASP. I do not mind it, it was ok 15+ years ago I guess. But now a days building a restful API is easier with the tools that we have. But nooooooooooo we still chug on that classic ASP. Funny enough, the lead developer for the company that contracted me(the ASP guy) does not seem to have any desire whatsoever to change stacks at-all. So everything is slow as shit, it gets better once we put work into it but yeah, definitely going against the flow here.