Different routes can be served by different servers, if the bottleneck is in CPU usage.
Different async tasks can run on different task runner services, if the problem is tasks competing with each other.
Different test suites can run for different sections of the app, if the problem is with tests taking too long to run.
Github and others even allow specific subfolders to be "owned" by different teams.
What else is there? Even slowness of compilation and/or initialization can be alleviated, depending on the language or framework.
I'm talking about how monoliths can also fix such problems when they happen.
In truth, they're superficial technology terrorists.
There's tools that make it easy, I'm still using a tool I built 10 years ago. Very little has changed except the addition of layers, which are also pretty easy and automatically handled in my dev tool.
All the Lambdas I write also run locally, and testing isn't an issue.
The only gripe I have with Lambda is when they deprecate older nodejs versions, and I am forced to update some of my Lambdas to run on current nodejs, which then leads to refactoring due to node module incompatibilities in some specific situations. But those are really nodejs problems and not so much Lambda problems, and it does get me to keep my apps updated.
YMMV.