Mine was always sluggish too. I took it off, cleaned the commutator and replaced the brushes. It made no difference. A few months later the brushes stuck. I had to remove the starter again to free the brushes and make sure they were moving freely. The next year it did it again
This time the brush on the earthing side had more or less welded itself to the brush holder. So bugger it. I went out and bought a new starter from Electrex. With it fitted the bike has never turned over so quickly, seems like twice as quick.
I came to the conclusion that the original starter must have an internal short in the armature windings causing it to draw too much current and reduce its power.
