Previewing pictures with Quick Look is very slow (even for small / normal size pics)

Dear Community,

Previewing pictures (even 350 KB) with Quick Look became very slow recently: I get the spinning wheel for a second before each pictures.

HW: iMac 2017, Intel Core i7 3.6Ghz, 32 GB, 1 TB SSD with 380GB free space

SW: High Sierra 10.13.1. No running apps in the background.

Simple restart did not help.

Anyone with the same problem?

Would be great to hear some suggestions.

Thanks in advance,

Attila