avconv performance slow in Ubuntu 14.04 trusty

I noticed that my TV conversion scripts were suddenly running very slowly after upgrading to Lubuntu 14.04.

After some investigation it turned out it had started defaulting to using libx264 encoding when the output file was named *.mp4.

When I ask it to use mpeg4 encoding instead, it goes fast again:

avconv -i MyProg.ts -acodec copy -b:v 2500 -c:v mpeg4 MyProg.mp4

I assume in previous releases it was defaulting to mpeg4, unless there's been some huge performance bug introduced into libx264.

Interestingly, when I built the latest version of avconv from their git repo, it didn't include libx264 at all, and happily defaulted to mpeg4.