Canon EOS 90D

[ChuckyA]

The grey market 18-135mm stm lens appear quieter and smoother than their US market newer? 18-135mm couterparts which make a sliding noise and are not silky-smooth when zooming. Is there a difference optically in the performance between the two? Is the US lens motor better despite the noisier zoom?

[kvbarkley]

I don't know that anyone here would have enough experience to answer.

[MikeSowsun]

Are you asking if a Grey market 18-135 STM is quieter and smoother than a USA market 18-135 STM? 
(or are you comparing the 18-135 STM with the 18-135 USM)

“Grey” market is just a marketing tool, not a design spec. There should be no difference as both lenses are made in the same factory to the same specs. 

[ebiggs1]

There is no difference except where the lens is delivered to. Of course there is always minor tolerance differences between any mass produced product.