A delete test that creates its contact through the form takes eight seconds and can fail for reasons that have nothing to do with deleting. The same test, arranging through the API, takes under a second and only fails when deletion is broken.
Multiply by a suite and this is usually the single largest speed win available. It is also a design decision that goes wrong in a specific way, so it needs a rule.
The rule: arrange through the API, act and assert through the UI.