I differentiate between actual "troubleshooting" and PC clean-up (or 'tune-up', 'speed-up', or what it more accurately is... clean-up).  This cleanup routine is fairly structured, depending on your version of Windows (only), and relatively cookie-cutter.  This is a flat rate service, and will not matter how long it takes (5 minutes or 5 hours) & all done remotely.

The "tricky" part with ANY PC cleanup effort is, what is causing the slowness might not be all the buildup of junk on the PC that I would be cleaning up.  It is possible that afterwards the PC is still sluggish.  However, with enough time I can at least identify the root cause for further troubleshooting and/or repair (separate service).