It's true that there's a non-zero cost for each test, but overall I think tests speed up development rather than slow it down (unless you go crazy with the tests, and providing you're fairly decent at writing tests). I don't believe it's worth testing all paths through the code, but I'd aim for significantly over 50% coverage to have any degree of confidence in the codebase.
I estimate I write about 2:1 unit tests to code in terms of tests to functions but tests should be quite a bit faster to write than the code they're testing. I think I'm at the low end of how much I test my code compared to other engineers, however.
Perhaps it is different in game development. One of the big advantage of writing tests is that you can aggressively refactor with confidence; if you're planning to stop improving your codebase once the game is released maybe this isn't an issue? Plus bugs are perhaps less of an issue if you inconvenience the gamer rather than lose someone cash, and maybe you aren't expecting to hand code over to new developers.
