The reason it's slow is not because of the rotation, it's because you reencode the video.
When you don't specify an output video codec, the default for mp4 is H.264 (libx264) preset medium, which depending on the resolution and your hardware, can be slow.
I see you try to go around that by specifying "-c copy", but you can't : rotating the video means modifying it, so there's no way can just copy the H.264 stream. Reencoding is not an option in your case.
You can try with one of the faster presets :
ffmpeg -i in.mp4 -vf "transcode=1" -vcodec libx264 -preset veryfast -acodec copy out.mp4
But the quality/filesize will suffer.
See https://stackoverflow.com/questions/25031557/rotate-mp4-videos-without-re-encoding, there's an interesting answer about changing the metadata so that players can rotate the video.