I think of "preinfusion time" as the time it takes for the basket to fill with water and become pressurized.
With my gear-pump modified GS/3, I lower the line pressure to 1-2 BAR and preinfuse with the pump off. I don't see drops on the bottom of the basket until the basket is full and begins to pressurize. There are two ways I can tell when preinfusion is over: 2) When drops appear on the bottom of the basket, and 2) When the GS/3 boiler gauge begins to rise from wherever it was when preinfusion started.
If I don't turn on the motor immediately, I call the next segment "preinfusion soak" time. Yes, drops are coming out, but the pressure remains constant at line pressure. Actually, I never do this, but it's a possible profile.
Once I turn the motor on, preinfusion is over. I call the time it takes to reach maximum pressure, nominally 9 BAR, "ramp time". I can control the ramp time with the gear pump motor speed.
On a stock GS/3 AV, and many other non-profiling machine, the pump is on from the start, so "preinfusion time" would be the time before pressure begins to rise, which is also the time before the first drops appear. Even with af flow restrictor (gicleur) inline, it's so fast that I don't think of it as preinfusion at all. "Ramp time" is determined by the pump and gicleur.
The stock GS/3 AV has a preinfusion feature that I never use. As I recall, it turns on the motor for a predefined amount of time, which probably should be the time it takes to fill the basket, and you can optionally program in a motor-off "soak time" before the "ramp time" begins. To me, this isn't really preinfusion because the flow rate when filling the basket is way too fast.
I believe "preinfusion time" on the Slayer is the same as my modified GS/3 -- time it takes to see the first drops or the first sign if pressure rising. AFAIK, the Slayer has no concept or capability to do a motor-off "preinfusion soak". Since the pump is on from the start, once the basket fills it's the start of the "ramp time", which is determined by the motor speed and needle valve settings, which are preset before the shot, not varied during the shot.
My sense is that first drops or first sign of pressure rise can be used to measure preinfusion time on most, if not all, pump machines. I've never used a lever, so that may be different.