1080p resolution is not what makes encoding slow. It's the specific options you select for it. If you set it to high bit-rates your encode speed will drop (i.e. if you encode a 720p at 1 to 2 MB/s, the encoding is actually fast). Likewise if you set in special options such as CABAC or setting the motion estimation method to something like uneven hexagon with the field search at like 64, your encoding speed will drop like a rock (the more advanced options you turn on, the slower it gets but the better the outcome).