Ok - I got it figured out I'm pretty sure.
I'm kicking my own *** for forgetting the first rule of Internet Club: "Don't believe everything you read".
So I decided to take advice and programmed Litchi for a fast lap around a school yard nearby. Set the POI's, set all speeds wide open (30 mph - yes I know it won't go that fast, overkill).
Loaded up fresh battery, made sure mission parameters were to RTH after done. About this time a 5mph breeze kicked up. Launched the bird up to 120 feet, changed to waypoint and verified parameters and started it. At no point would it go past 10 mph. When the mission finished it stated the mission was over and just sat there. Well crud... so I put back in FPV and set RTH. It returned all right but climbed to about 500 feet and sat there. NO throttle response - in fact it started drifting with the breeze. Well now I'm starting to panic when it gets 1/4 mile away. I terminated Litchi with prejudice and fired up DJI. Whew - I have throttle but only 5mph top speed again. I tacked against the breeze and finally got it down to 150 feet or so and was able to make progress home. Made it with 28% battery.
Went inside, changed underwear and was pondering what else had changed.
I remembered changing EXP settings on the recommendation of an "expert" for smooth video.
Naaaaaaw! That couldn't be it - could it?
First one is Throttle up/down - .20.- (left stick)
Next Rudder left/right = .2 (left stick)
Next is Forward/Right - Back/left (right stick in Mode 2) = .2
Indeed the settings make for a smooth camera - but that first one - throttle up/down is the culprit.
My understanding is the curve is a rate of change - but apparently it acts as a hard limit as well as rate.
I set it back to .9 max and wahoo! it flies fast again.
So I'm going to draft an email to Litchi about their app somehow affecting the MP to allow it to drift and became unresponsive.
I know the DJI app keeps logs - I'm gonna see if the black box has anything for the Litchi flight - anyone know?
