Benchmarking Spring Applications, Multiple Small Services vs. 1 Big Performance

I had a discussion of many small Spring services versus 1 large service.

What exactly -performance-wise-will be affected by these different approaches and how can I compare it?

In my opinion, attaching an extra class will only affect the startup time of the application and the extra time for a separate method call is also negligible, so I would choose a lot of small services as small performance differences do not affect clean code.