In ATM10 YACL's WEBP Preloader accounts for upwards of 15% of the total game boot time. The only two mods which use YACL in that modpack are BridgingMod and Variants&Ventures.

Granted, both of these mods do ship WEBP files, so the question of if the onus is on those mods or on YACL could be debated, but the preloader cost here is simply unacceptable. At a minimum you should ship a config option to skip the preload step.

In the short term we will probably be shipping a mixin to disable the preloader wholesale until a native solution is implemented.

If I managed to optimise the loader significantly would this be enough for you?

I'm following standard Minecraft practices by using resource reloader, meaning resource packs work.
If I can't get it fast enough, I will add a toggle to prevent this.

I mean, you could try, it probably still isn't worth it though. There's no point in paying a heavy startup cost to preload images for a config gui that less than one percent of players will ever open.

It makes much more sense to load them on demand