Hi I am glad that you have found the “culprit” now it should be easier to fix.
I am guessing a bit because I have not explored grease pencil and do not really know about fill layers etc,
Fill layers are defined by a loop of surrounding points.
In the manual it says:
“Precision: Multiplier for fill boundary accuracy. Higher values are more accurate but slower.”
This precision or resolution seems similar to the concept of curve resolution or in modelling subdivisions and if you set it too high you will defiantly get into problems. The higher the precision the more points it would create. I suspect that when you created the fill you gave it loads of precision without realising that it would have a negative effect.
The fill layer has its own set of points so you could isolate the fill layer with the button in the objects layers tab and reduce its vert count (try select- alternated, then dissolve) without affecting the lines. If you have extremely dense points you may get away with doing this more than once.
There might be a better way to do it, and precision it might not be exactly what I am thinking, there could be a precision factor that you can lower after creating the layer (I have no idea).
If you still have problems I would start a new thread and put Grease Pencil in the title that way you will attract the attention of people who use Grease Pencil.
“HELP! Grease Pencil fill layer has 7 million points!”
Edit: I did not take in this bit:
I had a look, if you are in curve editing (strokes are like curves) you can reduce the precision/resolution with this:
If you uncheck curve editing you can access and dissolve points but the curve editing parameter is a more correct non destructive (you can always turn it up again if you want) and easy way to do it. It will automatically reduce your points count