PET-G under-extrusion after changing the nozzle to a 1.2 mm d. one (Prusa i3 mk3s and PrusaSlicer v2.0) PET-G is the main filament I use. I can achieve high quality of my prints with my Prusa i3 mk3s printer. Recently however, I've recently changed my nozzle from the default 0.4mm to the 1.2mm. I watched the online video tutorial on how to do it properly and did the whole process carefully. 

Temperature: 250&nbsp;&deg;C/100&nbsp;&deg;C
Speed: max 200&nbsp;%, but results are the same even all the higher ones&nbsp;.
Layer height: 0.35&nbsp;mm

With my first print (and the next ones too) on the new nozzle I noticed lots of under-extrusion with infill (holes). LOTS of stringing although I've increased retraction. What's the reason ?
Piecing this answer together from the comments on OP's post.
Myself and R.. noted that a layer height of 0.35&nbsp;mm, nozzle width of 1.2&nbsp;mm, and a fairly high feedrate (200&nbsp;% according to OP, no reference to what 100&nbsp;% is), is an exceptionally large amount of plastic to try and melt through almost any hotend on the market except possibly the Volcano, or the high-throughput version of the Mosquito. 

OP responded that lowering the layer height to 0.2&nbsp;mm seemed to fix the issue. That's still an impressive amount of plastic, but it's a 43&nbsp;% decrease in total flow compared to 0.35&nbsp;mm layer height.