when does a single monolith become a bottleneck

my project is basically one giant app right now. it was super fast to start, and we got a lot done early on. but, we're hitting slowdowns and coordinating deployments is getting really messy. i'm wondering when you all decided to break things out. what made you think about microservices or a distributed setup? like, were there specific things you measured or problems you ran into that made you do it?