If you remove the back panel you will be able see the drum rollers. There's a good chance they may be worn out. As long as you are replacing the rollers I would suggest replacing the belt at the same time.
Typically when the rollers wear out, the drum drops ever so slightly and the drum wears on the front and rear drum to cabinet seals.The added drag can cause a slow drum and squealing noises.
Good luck, let me know if there is anything further you need.
Regards,

Your belt is slipping, replace belt. While apart check rollers, if the are worn, meaning wobbly or loose replace also.