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?
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.