Add --stats, any install, run your own benchmark.
I did some checks comparing gzip vs. bzip on metadata repositories sometime last year. IIRC, I posted the results to this list.
The result was not as eye-striking as one might expect.
Benchmarks are in the eye of the beholder. My 5-7x slower was done in rpm-3.0.5, immediately after putting bzip2 into rpm. I have had no reason to go back and recheck the data.