Verified sd-card / reformatted via tool from https://www.sdcard.org/downloads/formatter/
M39: SD card in slot 0: capacity 31.72Gb, free space 31.55Gb, speed 20.00MBytes/sec, cluster size 32kb
From ideaMaker settings I have changed minimum segment length from .01mm to .1mm, yielding no appreciable change in look_ahead underruns.
Sliced the same part in PrusaSlicer, yielding no to ~5 look_ahead underruns whereas ideaMaker yielding 50-60.
All the while, the part is coming out absolutely fine - no defects.
My conclusion can only be that look_ahead underruns are not critical. Simply a number representing the number of times that RRF has 'optimized' the gcode, amending unneeded deceleration-only moves, before the physical planning process.
@dc42, Please correct me if I am wrong.
Cheers,
Kolbi