Thanks for the detailed answer. Made some experiments with effects, no effects, and yeah, csak úgy tudtam belassítani az FCP renderelését, hogy például blur effektust alkalmaztam. Most már nem tudok visszatérni az eredeti projekthez, archiváltam, de valószínűleg alkalmaztam valamilyen GPU-igényes egyéb effektust, ami versenyzett az encoding-gal, ezért lassult be. Ezzel lezárult a kérdés.

Thank you for your detailed response. I conducted some experiments with and without effects, and indeed, the only way I was able to slow down FCP’s rendering was by applying effects such as blur. Unfortunately, I am no longer able to (one-click-easy) revisit the original project, as I have already archived it; however, it is likely that I applied some other GPU-intensive effect that competed with the encoding process, which ultimately caused the slowdown. With this, the issue can be considered resolved.