My oldest af lens is also the 300 f4 non is. Bought for 2nd hand. Not sure is it the age or my body focusing might be a tad slower than other usm lens and there is this faint creak sound when the af motor is running. Hope it doesn't fail on me either. Other than that it us working well still.
focusing slower than a USM lens should be normal, but it might be abit slower due to drying of the grease or dust getting into the moving parts. a good CLA should boost the AF speed (did for my tokina 300 2.8 and nikkor 300 f4 1st gen and afew others)