>Over time you realize that testing truly does not slow down development as much as many people think it does.

Not my experience. We just built a new codebase, rewriting an older project with typescript and all the modern libraries and conveniences. We spent about 2x more time writing the tests than we did any of the API code.

Tests can be so fiddly and not exactly straight-forward. It takes a lot of time, but that isn't a reason not to do it. But don't suggest it's going to take less time, even in the long run, because it isn't - you essentially have to maintain 2 codebases now, one for the actual code, and one for the tests. Both are points of failure and both can be a time-sink.