That was definitely the culprit, thank you! For some reason, the default value was too long for me and caused sort of distraction. Now that I've changed the parameter it feels way smoother, I see thumbnails instantly, and that's exactly what I needed. In case anyone may be interested: I set my value to 15 ms. For me that's a comfortable option.
I left StartAllBack 'precious little animation' on, by the way. Nevertheless, it feels good to know how and where I can disable the animations any time I might need that.
StartAllBack is something that really helps. I tried Start11 as well, and that one couldn't even maintain correct spacing between tray icons on a two-monitor setup with 2k laptop screen and Full HD AOC monitor, and it looked just awful.