That depends on the encoder and the settings you use. My own playing around with CUVID and NVENC support in ffmpeg a couple of years ago worked out quite well.

That depends on the encoder and the settings you use. My own playing around with CUVID and NVENC support in ffmpeg a couple of years ago worked out quite well. I found that I could take 1080p H.264 footage, decode it, overlay a text display, and re-encode it back to H.264 much faster than real time without perceptible-to-me quality loss. I saw no problems with the video quality if I used -profile:v high -preset:v slow and set the output bitrate equal to the input's average bitrate. With those settings, I was able to reencode at about 130 fps - handy when the raw footage was 9+ hours at 25 fps. Yes, that's "slow" on the GPU. :)