I have no troubles whatsoever on a S905X with 100MBit and a S905 with GigE. 4k content are live dvb-s2 streams, other content is high-bitrate mkv files from a samba server.
Please tell us more about your setup. What box are you using, where are your files stored? What network do you use? Is there a sample file that repeatedly triggers the buffering that you could provide?
Also, I can recommend to "start fresh" with a new install on a spare sdcard without any changes to configuration, to rule out any issues with your settings.
can you share you DVB-S2 device Model and if it possible to send me Link to it