On my garmin edge 830, the speed drops when under trees.

Installing a speed sensor will eliminate the speed drops.

Maybe you're not pedaling as hard when under the trees.? :-)   I've always used a speed sensor with Garmin devices because it's much more reliable for speed and auto-pause, particularly on tortuous tracks.  It used to be you had to faff with a magnet on the spokes and getting it aligned to a sensor on the chain stay.  Now it's just strap it on the hub and go. There are inexpensive ones available on Amazon etc...

Thank you for time and answer, so I had to pedal a little harder :-) no I will buy a speedsensor

Thank you for your answer

Mine started doing this after the last firmware update.
I have several bikes...some have speed sensors, some dont...but the speed and auto start were always fairly close on both during the summer with heavy leaf cover.
With the last update in late fall....I started seeing the speed and auto start/stop issues.
Ended up putting speed sensors on most of my bikes

Same experience with me, I had no problem for years with the edge 830 and suddenly this occurs. Something to do with planned obsolescence?

bowie70 can You share a FIT file here (or in pm) for raw analysis? 

I can't attach a FIT file to this chat