avconv huge output file size

Question

I am trying to rotate some mp4 videos (shot with an Android smartphone) 90 degrees clockwise. I have been using variations of the following command:

avconv -i 20150604_192846.mp4 -c:v libx264 -preset fast -crf 23 -vf transpose=1 -c:a copy 20150604_192846_flipped.mp4

with the variations being in the -preset and -crf settings. I have tried with -preset as both medium and fast, and have tried -crf as 18, 23, and completely excluded. The input video is about 1:30 in length and has a file size of about 200 MB. No matter what combination of settings I've used, the output video takes hours (as in overnight and still not done) to encode and ends up being GBs in size.

What is going on here? How can I simply flip my video 90 degrees clockwise and keep the size reasonable?