My computer can run an ARAM teamfight in highest settings with over 100 FPS. Sometimes in Onslaught it drops to two frames a second, in mid quality, and I'm not sure that's my laptop being who-knows-what. The game demanding gigantic jumps in performance (technically spikes at over 50 times the highest demand in ARAM) isn't normal and my frame rate crashing down when I least need it (third form Kayn, tons of monsters) is some undeserved handicap.
I played a number of other games that were limited, not by your PC, but by the game's rendering engine. You could have the highest performances in the world and the engine would not make use of them, by being gated in its program to using only 1 Gb of RAM at most for example. In LoL's case, I'm unsure how it works, but I'm pretty sure no one in the development team was really concerned with augments permitting e.g. Yas' tornado map-wipe (two Last Breath augments, can hit 30+ monsters with the small cytes spawning thus release 60+ tornadoes). Maybe they didn't think about it, or they thought it wouldn't be a problem, or they didn't have the time / resources / means to downgrade graphical effects when the performance demand spikes, or maybe they didn't care.
In any case, it's just poor coding if you're expected to play this mode with 98% of your PC's resources unused until the storm hits, takes a hold of all your processing power for two seconds, then goes back down to 2%-ish resource usage. It's like a rock concert with speakers' volume so low you had to hold your breath to hear the music, and suddenly everything jumped to maximum power for two seconds, then dropped down to minimum again. Your ears would be bleeding before you even understood what had happened.
Also: volcanoes deal absurd amounts of damage, and when your screen is obscured they won't appear to you until you set foot in one. That can be a good 500 true damage if you instantly step out, but if anything kicks you into it (anchortail, razorheart, hogtongue, dozermaw...) and you're killed mostly because the volcano popped up from nowhere, I dare say you don't lose because you were bad; the fault lies mainly with the fact that you couldn't see something that was extremely important. Imagine Kayn turning invisible when doing his Q dash-and-slash, that wouldn't be any less fun.
This is an example of fake difficulty, because of critical information being denied. Other examples could be the fact that monsters gain more damage and HP (artificial, it doesn't change much to the way you play); some maps are extremely binary (right comp, win; wrong comp, lose) and you have a chance of encountering local wildlife that you won't be able to take down, no matter how well you play; the boss fight has a timer represented by unavoidable damage over time once you exceed the limits of Kayn's patience.
It's all pretty unnerving, and while it does contribute to making the fight difficult, the are other levers to pull that could do the same. Lower difficulty settings could feature less wildlife diversity, with e.g. only alphacytes, shimmerstrikers and arcraptors in the Intro difficulty, then more species up until Onslaught. It could be Kayn growing more active, with e.g. a cooldown time on most abilities in Intro level that are decreased / removed in higher ranks. The difficulty of objectives could increase gradually, with intro as "just kill stuff" and Onslaught with the more elaborated "look for Ora blossoms", or even some king-of-the-hill-style thing like Star Guardians' moving safe zone. There are possibilities, and a simple increase in damage and HP is really just tasteless. So now I'll need twenty hits instead of ten to slay that alien thing? Why not a hundred, or a million? It would be more difficult, but not more fun.
That said, I could use a hand in completing the Universal legends mission, as well as the Morning Star veterans one. If you have time and will this week to help me out, then I'll be most pleased. I'm afraid I can't give anything in return, though.