There's no doubt that computers slow over time due to a number of reasons. The hard disk becomes more fragmented. The startup can become clogged-up with nuisance programs. Various cache areas (such as temporary internet files) become full. The registry becomes larger. Merely playing (and installing and uninstalling) quite a few games should not cause these issues alone however a good tidy up often helps things. A friend rebuilds his PC once a year to avoid this and a freshly rebuilt PC is always quicker. Is he going to rebuild your machine or simply remove some files?
If you do go ahead, and the price is reasonable, I'd be tempted to time some aspects before and after to check how much he has improved it ! I think a fresh rebuild of the machine is always the best option.