To get it out of the way early, no, this is not related to low frame rate causing low ROF due to the limit on fired events.
This has been tested from 60fps to over 200fps with the same results.

Problem seems to be that Arma is not correctly calculating how much time has passed between shots and sometimes delays the subsequent shot by an extra frame or two.
This slows down the rate of fire of weapons, on average by 8.47%, but with an extreme reduction of up to 22% for some weapons.

Test data can be viewed here: https://docs.google.com/spreadsheets/d/1cfCJ9TggKT75_aJHGzGO2sj9yxAd_OdxfXB8fCuCOpA/edit#gid=1025499901
The script used to retrieve this data is here: https://gist.github.com/Drofseh/298d3dfb5245a9d5258772b66db9114a