Some tests are slow, but even if each test is fast, they can easily add up to a lengthy time if you have a lot of them. This is especially true if they can't be parallelized and need to be run in sequence, so you should always aim to have each test stand on its own without any dependencies on others.
It is good practice to divide your tests into rings of importance so that you can at least run a subset of the most crucial ones on every CI build. However, if you have a large test suite or some tests that are unavoidably slow, then you may choose to run these only once a day (perhaps overnight) or every week (maybe over the weekend).
Some testing is simply slow by nature and performance testing can often fall into this category, ...
