Almost no one care about how precise/reliable the keyboard pressing is, here, we built a damn cool equipment to examine this question. Broadly speaking, the time precision is good, mean variance is less than 0.2 ms.
Environment: MATLAB with Linux (Ubuntu 14.04)
Monitor: SONY Triniton at 120Hz
Keyboard: Mechanic number board
In short, the delay is about 12 ms and the variation is quite small.