Circular saw doesn't cut properly
After the saw has failed to complete a cut quickly unplug the power, remove it from the work piece, and test whether the blade still turns freely. That'll check whether there is some drag that develops while the saw is in use. If so, repair might not be economical.
Assuming that it's a normal AC or universal motor (ie, not a brushless DC motor) then it's worth taking a few minutes to inspect the brushes. A motor can start acting weird when the brushes are nearly worn away. A new set of brushes is usually an inexpensive repair.