401s are not the fastest running machines but they should run at a reasonable speed. I would suggest using denatured alcohol and cleaning everything that moves internally. Then clean the oil out of the motor. If you don't want to disassemble it, get some electronic equipment cleaner and hose it out. Pull the brushes and springs if they come out and clean them too.
Then put it back together and oil it properly. Don't forget to grease the gears too. The owners manual has instructions on where to oil and grease, so if you didn't get one Singer has free downloads.
Joe