Very serious frame drop

Those log lines means that Shotcut has found and added that filter to its filter chooser menu. It does not mean that it was added to any clip. But yeah, HQDN3SD is slow but also is “zooming” as reported. In any case you are correct about some filters being demanding on CPU.

Basically, this topic is in the FAQ under “Why Is my shotcut playback so laggy.” There are some pinned posts at the top of the forum telling you to check the FAQ. It is also related to some info on the web site FAQ page. In addition to the remedies mentioned, this frame-dropping behavior also depends on Settings > Player > Realtime (frame dropping). Turn that off, and it does show every frame but then your audio might become more choppy on sections that require heavy processing (because it is waiting for video to catch up).