I commented out all not necessary entries in /etc/fstab except / and /home.
Then the speed of navigation in dolphin was very fast!
Then I reactivated the mounts one by one, with restarts of the system in between, until the speed was going bad again. With following two entries I could destroy the perfomance of dolphin simply by uncommenting them. No system restart not even any mount/umount action in between. Only editing of fstab did the difference:
LABEL=EXT_HD6 /srv/nfs/m/h6 ext4 noauto,users,noatime,nodev,nosuid,noexec 0 0
LABEL=EXT_HD7 /srv/nfs/m/h7 btrfs noauto,users,noatime,nodev,nosuid,noexec,compress=lzo,subvol=@ 0 0
I do not see anything wrong with those entries. AFAIK noauto entries should not have any negative effect, at lease as long as they are not used.
BTW: I have a lot of noauto entries in fstab for all sorts of external USB sticks or SD cards or external HDs, in order to have them always on the same subfolder.
Most of my noauto entries were left commented, so the difference of above entry enabling was not that big as with all noauto entries enabled, but it was about a second delay.