While perusing the upickle documentation I noticed that in one of the patch notes later than the version you're currently using, the phrase "~ 3x faster" appears: http://www.lihaoyi.com/upickle/#VersionHistory

You can get in on that by upgrading :D

Also for the record despite the issues I've raised the library is pretty handy, thanks! If I get time I'll look into submitting some pull requests; at least two of the issues I've raised should be trivially easy to implement.