Calculating whether a particular roll is a critical success in Basic Roleplaying and its derivatives (Runequest, Openquest, etc.) is probably the most time consuming part of resolving a roll. Depending on the version, one needs to figure out what 10%, or sometimes 20%, of one's skill score is and then whether the roll comes under that number. While one can precalculate the number, the actual skill score frequently changes due to bonuses and penalties, which also change one's critical threshold.
I propose that adapting the method of resolving critical successes from the Harn system would allow one to resolve these rolls more rapidly. I'm surprised this hasn't become a core part of the BRP system's resolution. Harn's system is also percentile based, and you achieve critical successes 20% of the time, but the system can be easily adapted to the 10% threshold I prefer.
If a roll succeeds, and the ones digit on the roll is a "5", the roll is a critical success.
This speeds things up by removing a process of calculation and replacing it with simple recognition. Choosing the "5" digit has the same effect as the already existing rounding rules for critical thresholds.
For a 20% critical success threshold, the two digits should be "5" and "0".