Am I to assume it worked normally new and recently started getting slow?
I would wonder if you have capacitor failure and it's starting to get instable. This is fairly common after a few years, especially on less than top tier sets. Assuming the power board is separate from the logic board, they'll most likely be caps on the power board and have visible doming/leaking tops.
If you have a basic background in electronics and a soldering iron you can DIY replace the caps with far better ones than came in it from the factory, for under $10 shipped from electronics supply houses like Digikey. In particular you'd go for very low ESR type capacitors for example at Digkey the Panasonic FS series in the same size (lead spacing), at least the same voltage and capacitance (though larger values for these parameters won't hurt if the model # still fits in the available space - no taller nor larger diameter than the originals to avoid clearance issues).
Then again if the sluggishness corresponds to amount of time being turned on (heating up) could be you have a bad solder joint, or transistor (discrete or in some IC) starting to fail.
Considering the warranty is over (yes?) I'd pop it open and see if there is any visual clue, right after trying a factory reset that PuppettMaster001 mentioned if that's an option.