My turntable sounds like the speed is too slow

Allot of great advice already so the only thing left is what the TT is sitting on or next to may be contributing to you problem?

Playing a 45RPM record at 33 1/3?

The cell phone apps for turntable speed are not accurate.  In my experience they read fast by about .3 to .5 rpm.  If your cell phone app is reading 33.33 chances are your ears are correct and the TT is slow.

Another way to check speed is time the length of the recording and see if it matches the stated time on the sleave.

Playing a 45RPM record at 33 1/3?

I bet I'm not the only one with a new and unheard record who forgot to change the speed and thought it sounded strange...

If you don't trust your cellphone app, maybe mark the platter, play a record and use the phone's stopwatch function to time 100 revolutions which you can count. Should take 180 seconds. If it does, time to look downstream starting with the stylus, the cartridge the mounting geometry etc.