thought that would be helpful to share my experience so far. Got some hints from ChimeyJimmey to adjust advancedsettings.xml. In fact it turned out I was using Kodi`s default cache which is 20MB.
So I tried all kind of settings, right now I am running with this config in advancedsettings.xml
So this is almost 140MB of cache, which requires as per Kod`s wiki x3 the amount of RAM, i.e. 140MB of cache requires 420MB of RAM.
The default in OpenELEC for the Pi2 is 256MB.
I've found that when running the GUI at 1080 with high colour depth and higher quality fanart/imageres settings in advancedsettings.xml, gpu_mem=320 is necessary to eliminate resource issues.
So I set my gpu mem to 320:
LibreELEC:~/.kodi/userdata # vcgencmd get_mem gpu
At the end now the Rpi3 has 688 MB memory left (420MB of it can be used for cache) leaving 286MB for Kodi which is... well... tight.
Anyway - now all the stutter and lag is gone, even when streaming 40GB mkv files from my NAS.
I guess that my memorysize is not ideal, but I will have to tweak it to get to the lowest workable for my box and network.
Anyway - wanted you to know - now it`s solved!
p.s. It`s interesting what the change to default cache is in Kodi 18, I guess the same? Then it`s interesting why in Kodi 18 I do not have any issues, even do not have to create advancedsettings.xml to increase cache and increase gpu mem... let`s see.