Dreamcast glitchy/badly lagging when powered on
When I turn the dreamcast on, the video acts like it has a very slow FPS and is very slow starting up. It still works and plays disks, but lags badly.
Sounds like it's possibly overheating. Listen / feel for the fan when you power on the system. You can also try replacing the thermal pads inside the system (they can wear out with age). You can buy the fan or thermal pads on ebay (cut-to-size). There are several guides on this site that show how to disassemble the DC and where the thermal pads are located. It's pretty easy.
Just as a side-note, if you're playing burned games (no judgement), they can sometimes behave strangely (running slow, glitchy, crashing, audio problems, etc). I recommend re-burning on a different brand of CD-R