Read speeds, bottleneck, correct understanding?

Hi,

If my program reads through an LMDB database apparently slow at first run, but suddenly runs much quicker at the second run (which does exactly the same thing), can't I already say with confidence that my choice of LMDB - in this case its builtin memory mapping functionality - is already paying off in terms of read speeds?

I mean, that alone (exact same thing runs much faster the second time) already PROVES that the bottleneck was reading from disk as opposed to memory, right? I'm looking for excuses for patting myself on the back for investing in LMDB.

Regards, Sam