So, a restart solved the problem related to the AV software. However, it did not solve the very slow writing to disk by SNAP. This time (after increasing the buffer), the writing to disk took about 18 hours (339 Mb).
I don’t know if this problem is related to this thread, GeoTIFF-BigTIFF writer is very slow when saving metadata
but since it is storing the Meta-data here as well, this might be the problem. However, I don’t if there is a way to disable storing of meta data, when saving in the BEAM-DIMAP format? If i do a new subset of the previous subset, the saving process takes like 10-30 seconds (and the size is about half the previous subset, 18 hours, so the relation is really unreasonable).