I used to fool around with old dudes biatchin about their pushmower's cut and all. Turns out that the problem was they were running the mower @
2000 rpm or so.....
They refused to run at the advised rpms
If there is one of those cheap Stihl trimmers within several blocks running on a calm day here in town you can pick it out from a bunch of others by the noise they make running wide open all the time.
Running partial throttle and running without the shield/cutter is bad and voids Stihl warranties. I have been using an FS90 for a couple of years, and yes,
I might throttle down a tad when I get into small loose gravel, but otherwise, the thing is slow enough as it is.
I run my Stihl 90r and 110r wide open all the time, that's the way there designed to run. The only time you shouldn't or vary your speed is during the break-in period, the first five tanks of gas.