The trigger is always too slow, and that is fine
A return stroke lasts on the order of tens of microseconds. A trigger has to detect the flash, decide it is a flash and not a car headlight, and then ask a shutter made of moving metal to open. The last part is the problem: the metal is slower than everything else by two orders of magnitude.
So the trigger never catches the stroke it fired on. What it catches is the continuing current, and any subsequent strokes down the same channel, which on a multi-stroke flash is most of what you wanted. On a single-stroke flash it catches nothing at all and you have a frame of rain.
| Approach | Frames per hour | Keepers |
|---|---|---|
| Trigger, f/5.6 | ~40 | 3-4 |
| Open shutter, 6 s, f/8 | ~600 | 8-12 |
| Open shutter, 20 s, f/13 | ~180 | 5-6 |
The middle row is where I have settled. Twenty seconds sounds better and is worse: on a warm night the sensor noise creeps up, and any car that comes round the bend writes a stripe through the frame you were waiting for.
The keeper rate is not the point. The point is that when it does work you have the whole channel, from cloud base to ground, rather than the brightest third of it.
None of this applies in daylight, where you need the trigger and a heavy neutral density filter, and where I have never once got anything worth keeping.