Nic Jansma’s Performance Calendar post Compressing UserTiming is a great read for those interested in collectioning UserTiming data and compression in general:
Basically, if you have good domain-specific knowledge of your data-structures, you can often compress better than a general-case minimizer like gzip or MessagePack.
Source code is available at https://github.com/nicjansma/usertiming-compression.js.