I was wondering whether there exist some general ideas of the performance impact when using bigint?

I was putting together some snippets for projecteuler.net (yes, I was bored and uninspired). The 14th problem asks for the length of Collatz chains http://projecteuler.net/problem=14. Anyhow, I wrote that http://pastebin.com/jbYBFiRZ up.

Or rather, at first I had one extra line at the top "use bigint;". Removing that line improved the performance from ~10min to 6sec or by factor 100.

Am I missing something or is bigint incredibly slow? Thank you for your insights.