It depends upon many circumstances, so "fast" or "slow" is in two departmets, opinion and facts. Many deep divers don´t like being reeled in fast, when pulled by the reel they rest on their side and don´t run true, but you have to check your bait and see if it does it, if it does then don´t use a high speed reel for those. Deep divers can generate a lot of resistance, reel them in with a fast reel and it´s like trying to reel in a bucket, after a while between the drag and the vibration your arms end up toast. For baits like jigs, many spinnerbaits ( depends upon blade size and configuration ), soft plastics a very fast reel allows you to either work the bait fast or to pick up the slack in an eyeblink ( so you don´t have to reel in like mad ). On the subject of you can slow down, well, why worry about having to slow down when you can get a reel that does it for you ?