I apologize if this should be a separate thread, but I feel like my question might align very much with that of the OP's....so I am going to jump in here as well.
I also have found myself wanting to use entirely different amp rigs (e.g., dirty Marshall for lead, clean Fender for rhythm....or, different "Presets" in AX8 parlance), and so -- in an effort to operate my AX8 in that way -- I have sometimes configured my footswitches to operate in "Preset Mode." In this way, a single pressing of footswitch 1 yields one preset, while pressing footswitch 2 yields a completely different preset. While this method works well in general, I have found the lag time in switching between presets to be too great (too long a lag) in order to effectively switch between lead and rhythm within the same song. This observation on my part seems inconsistent with Chris' statement above; I am wondering if perhaps I am doing something wrong?
I do find -- conversely -- that I have virtually NO LAG if I instead operate the numbered 1-8 footswitches to call up "scenes" within the same preset (e.g., press F2 and then any of the switches 1-8). I also have no lag if I am simply turning ON/OFF any of the effects within a given scene. Thus, I have limited myself to pretty using only one preset for a given song, simply stomping a numbered switch to turn on/off an effect when I have to play leads. This works reasonably well, but it is a compromise. I would prefer to be able to be able to simply operate in Preset Mode and just switch between entire presets....but again, I am finding the lag time to be too great.
This doesn't to square with the guidance suggested above by more-experienced users. In my case, operator error may be a factor. I will appreciate any insights.
changing Presets does have an audio gap. it's not a "lag." things are changing immediately.
there is an audio mute/gap placed there. if you change settings immediately without it, there will be noise, pops, clicks. any digital gear does this as the same sound coming in is suddenly changing hundreds of parameters. most gear - including real amps - place either an audio gap or a fade in when changing channels or similar.
engaging and bypassing blocks do not have an audio gap because they are already "on" just not processing audio at that moment. this is different than changing Presets or XY because
no settings are changing. it's simply engaging or bypassing.
if you change XY on the Amp block, there is an audio gap for the same reasons. this is important to know because many people say "Scenes have no lag/mute." but that's only if you don't use Amp XY.
i said earlier that Presets are the fastest and most consistent for changing tones. no matter the CPU usage, Presets will change with the same amount of audio gap regardless. so it's consistent. compared to Scenes changing XY, there
is an audio gap when changing amp types - which is typical of a Scene change - so it's not consistent. if the CPU is low, it may be a short audio gap, if the CPU is high, it may take longer as other things need to load.
the AX8 is doing an incredible amount of things with its single CPU. it's not an Axe-Fx II. they've squeezed so much into this unit.
even with many real amps, people have to change tones slightly before the down beat so there isn't a glitch, noise or gap. again, parameters are changing instantly with the same input audio running, there usually has to be some sort of mute. the AX8 is doing its best to change hundreds of settings as immediately as possible.
if i absolutely need to change from Clean to Distortion without any gaps, i use a Drive block that just engages/bypasses. otherwise, i stop playing guitar for an 1/8 or 1/16 note, change Scenes or Presets just before i need it, then play.