[LoRd_MuldeR]

h3nry, you need to be aware that "high quality", "low bitrate" (small file size) and "fast encoding speed" are three competing goals. You cannot "optimize" them all at the same time. You can only find a decent compromise/trade-off.

So I think you have to decide what exactly you want:

(1) If you want to hit a specific file size (i.e. a specific average bitrate), then simply use 2-Pass mode with exactly that target bitrate. Use the "slowest" settings you can afford to squish out the maximum quality at that size.

(2) If you want to retain a certain level of quality, first pick the "slowest" settings you can afford and then find the highest possible CRF value that still satisfies your quality needs. This will be as small as it can get for the desired quality.

(3) As others have said, with a "lossy" compression each re-encoding unavoidably causes a certain amount of loss ("generation loss"). Get an external USB-HDD drive, move your files there and avoid re-encoding altogether...