Thank you both!
Absorbed moisture in your PLA can do this - steam bubbles form in the hot end and cushion filament movement so it doesn't immediately result in extrusion/retraction. Also look for any slop in your bowden tube end fittings - if they've worn the tube where they grip it, cut a little off the end to get a fresh grip.
Would steam bubbles typically appear as a steady effect, or more as sporadic events? It seems that the filament "lag" is pretty consistent and reproducible for me. The filament has been on the shelf for a while, and I have not tried baking it. Had it stored under normal room conditions, not a damp basement or such.
The bowden tube ends look alright, I think. I had trimmed the tube a bit a while ago since it seemed longer than necessary, and hence had a stronger bend.
Before you get carried away doing a bunch of adjustments and tweeks, insure that your printer is properly calibrated. Do each of the X, Y and extruder axis as shown in this video. Especially step #3 since you're worried about the extrusion.
To begin a print, I have added some custom g-code to print a line of filament along the left side of the build plate to prime the nozzle. My Creality CR10 never prints the first little also. (For the Prusaslicer, this goes in the 'Printer Settings' tab.) Some slicers do this by printing a skirt first, which I never liked.
I checked the extruder calibration last year, and it seems good. Passed the test, and also I get nice layers as long as everything is deposited continuously without breaks.
As mentioned, the PrusaSlicer does do a priming trace at the beginning, with a higher-than-normal extrusion rate. It is long enough to get the filament flowing reliably, but it's noticeable that there is always a dead time at the beginning. I also let it print a thin skirt, which serves to take up the unwanted "tail end" of the extrusion at a higher rate, so it does not mess up the print itself.
The delayed onset and end of the "fat" priming trace do not bother me per se. But they are indicative of what continues to happen later during regular printing: Whenever the print head has to move elsewhere and interrupt deposition, filament supply to the nozzle continues after it should end, and then restarts later than it should.