Hmmp careful now!
Gatorgirl and everybody: The Mac will run slow when the disk is full - also with temporary rubbish - files created while you watch a video and you never see, because the OS will delete them. So if your disk is above 85% full - you have to do something, delete files and empty the trash. You need a utility to find duplicates, and quickly identify the files you do not need - like support for all the foreign languages, the cache files - and the problem is that deleting files also in the application cache can give you big problems, so a tool makes sense.
The problem is that one tool that is very much advertised and used by many is no more than spyware. The question here could be rephrased: "Is Detox just as bad as M??Keeper?". I use ClamAV, have paid for it and contribute also since they intend to develop it to also protect your browsing - not just the email scanner. It scans the files that the browser place, to see if this is malware, but this comes from Linux and based on the same security as on Mac.
Run "Disk Utility" - hold CMND+R while you boot, and make it verify and repair the disks.
This will report disk usage: and if more than 80%, you must delete files and recover space. When you verify and repair, you run through the file system, and this will find disk pages that is marked as "used" but really not used by anything and recover the space. If the verification finds errors, it has 7 back-traces that can be used to recover the disk, but you risk loosing the last 2-3 minutes. The system is remarkably resilient - use the system provided utilities to "repair" the disk. Even on an SSD, the time spent searching in the indexes can be dramatically reduced, and the spinning of the wheel reduced - because shorter search in the file system. This is a Unix property and has been around "always". The 85% is also well known constrain - it is not related to fragmentation, but pointers in indexes are used to say where "next page" is, and when searching for "next spot" takes time, the system becomes slow.