I would check the belt itself. My mother's machine was slow like that, and I ordered a new belt because the old one looked tired. The new belt was so stiff that the machine would not turn at all! I did some research on this, and found the Featherweight Shop, and they had just posted that "the new superbelts have just come in.." I watched their video, and ordered one. The difference was amazing! I have been using nothing else since. These belts are soft and flexible, and allow the motor to turn the machine easily. Watch their videos, and they're a great place to know about.
https://singer-featherweight.com/