The CDR and AP of Support Malphite R - Mid Game To Late Game and its snowballing

I know all of the basic stuff - don't clump so he won't R, don't get picked out... I was playing in a game as Rakan and Xayah as my ADC. We weren't extremely ahead but we weren't definitely behind either - and I would confidently say that we went even, borderline better out of laning phase. During my recollection of that game, although he ended up extremely fed, I didn't remember the Malphite support on the enemy team coming out of laning phase with an extreme lead. I'm positive my ADC had a CS lead too, and also their bot lane towerdived me like a dumbass and gave me a double kill.

However, mid game, this guy's R scaling and AP Ratios got insane. All he did was FLASH + R, and fuck we're all dead. I apologize for not having the reaction time, but it was when no one was anticipating anything. And also the hitbox is PHAT as hell. Suddenly, it was like he had no cooldown on his ult. Our team was relatively squishy, and one ult from him was enough to take practically EVERYONE out of the fight. Afterwards my team started to give him kills, but the problem was before it all happened - How did he manage to just completely frick us all over single-handedly with one R? Is that fair? He built one ludens and was able to half health my ADC with an R. Also he's not nearly as squishy as burst mages and has relative tankiness. I made sure not to give him any sort of lead in the early game and I knew I had done my job. How do you even play against this champion? I don't want to throw out accusations of him being broken or unbalanced yet.