It is possible to compensate out the effect of lamp turn-on delay:
But even without compensation, using a PH212 bulb, 20 1-second exposures are within 5% of 1 20-second exposure. The turn-on delay is in the order of 50 milliseconds - 1/20th of a second.
Although 'intermittency effects' are bandied about as the cause of errors, the real problem is in trying to use analog timers and metronomes as precision exposure controllers. Pseudo-digital timers - those with knobs to set the time, like the ZVI and Beseler Audible Timer - are little better.
The less said about cold-lights and this issue the better.