Let me start by saying that I use a 1920X1080 screen, so I dislike it when people post 720p. I generally have to actually go to YouTube rather than play the embedded file full screen. In YouTube I have the larger frame set to 1280X720.
I have never measured the export time for 1080 vs 720, but my guess is that reducing the frame size will require slightly longer export times. Check it for youself. Easy enough to do.
As for the bitrate, you could use the YouTube suggestions as a starting point. https://support.google.com/youtube/answer/1722171?hl=en
When using 720p instead of 1080p, Google suggests dropping the bitrate from 50K to 30K on the advanced tab, or for more "normal" video. from 8K to 5K.
Since the number of pixels drops from 2073600 to 921600, I am a bit surprised that they don't drop it to about 45% of the original instead of around 60%. But my guess is that they know what they are doing. Perhaps overhead hinders ding the math.