Firstly, you could most certainly change the gem you use to output explain information. The easy way if you have the data locally, is just to do "bundle open " and edit the gems code to do this. Otherwise fork on github, insert debugging logging in your fork, switch to using your fork instead of the gem.
Secondly, it seems very unlikely that it is the gem that is the problem, if an update really takes 70ms, with that SQL that you replied with, it should be easily reproducable in a local sql session, where you also can do explain analyze etc.
Do make sure that the query is not run exactly the same twice in a row though, since that second query would most likely be cached. I am not 100% sure how (do you use postgres?) the database handles caching in updates, but I think it could have an impact.
I would recommend you to google a bit for useful articles about using EXPLAIN and ANALYZE since they are great tools to know when troubleshooting SQL queries.
I suspect the machine you run this on is rather slow, or the database is capped somehow? Or it could just be that you have big indices set up on the table you are trying to update, which slows down inserts and updates.