There are a lot of problems with slow test suites.
During development you'll be inclined to run only portions of the suite (using testing-concept-focused-tests), which can lead you to overlook issues you've caused in other parts of the system when making your changes. You'll discover those problems once you run the full suite, but if you only run the full suite in CI (as you'll be inclined to do because it's so slow), then you'll bump into them very late testing-concept-prefer-local-testing-to-ci.
You'll be frustrated waiting around for the tests to complete.
You'll start ignoring occasional errors because the time it takes to fix the tests and run the suite again is too long.
You'll look to parallelize the suite, but that will cause its own issues:
This (testing things in parallel) has an obvious downside—doing your tests asynchronously is much more difficult than simply doing them in sequence. There are far more opportunities for bugs to show up in your test suite itself. You will need to weigh the difficulty of asynchronous testing against the time issues that can crop up if you test everything sequentially.
It will be such a pain to run the tests that you'll want to push them off to CI, but you should testing-concept-prefer-local-testing-to-ci.
"As more tests are added to the system, it could bloat even more, until running the test suite takes so frustratingly long that you start to ignore it or skimp on adding later tests, leading to bugs slipping through the cracks."