Thanks guys. I did open it up and the fans are running. I thought maybe the cpu was throttling due to heat issue or thermal paste, but it is like a slideshow so I wouldn't think the cpu would throttle that slow. Everything comes up, just very slow and it is unusable at that speed.
Once or twice, everything worked fine and the xecutor2 menu worked full speed and I ran doom on it and it ran full speed. But most of the time it is in this slow mode. I don't know the dashboard as it boots to the mod chip menu to run emulators and such. It is outputting via a composite cable to an old CRT commodore 1802 just to test with. This is a friend's system I am helping him with and the composite cable is what he gave me so I assume he is using that at his house.
HDD is connected with IDE cable. The cable looks good. The HDD spins up and seems to work ok. Maybe a cache issue or HDD space issue, but how would I check those things? I haven't worked on an original xbox in a long time

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I cleaned out the interior a bit with compressed air but the dust wasn't too bad. I took it all the way to where I removed the mainboard from the case and checked everything. Everything seemed in order. No burnt or broken parts.
I see the blue LED on the modchip board indicating HDD activity continually blinks every 5 seconds or so after the menu comes up, so maybe this could indicate something to do with the hard drive.