I was testing in single player a with stable framerate of 120.

Salvage beams are visible for 2.5 s when triggered by logic or short mouse clicks, but up to ~3 s when fired manually or with shift+click. Also, it's possible to mine blocks after the beam visually stopped for like 0.5 s. With an optimal group of 91 salvage modules I can mine 101 blocks with a single burst when controlled manually, but only 68 blocks when logic-controlled.

After about 1 min of continuous mining (holding down mouse button all the time) it becomes apparent, that the mining of blocks doesn't start at the same time as the beam anymore, but delayed. That delay seems to slightly increase over time, but the time it's possible to mine after the beam visually stopped becomes longer as well. The total mining time seems to remain the same.

Edit: Adding 91 cannon slaves results in mining ~135 blocks per 5 s. This is about twice as much as the 68 blocks from logic controlled / short click activation, so adding those cannon blocks is far worse than adding another group of 91 salvage blocks.