I might be wrong but assume the new client is on his final stage of testing, take this in mind before reading bellow:
To clarify i run this on both my computers but also talk with some other friends that play league to get their opinion. Both computers are recently formated with windows 7 Ultimate and drivers up to date and runs no problem in game or any other game, i got a desktop with AMD A10-7890K, 16gb Ram and Asus GTX1060 and a laptop with i5, 8gb and hd5670.
I will start by saying that the client looks cool, i dont enjoy much about the friend list and some other features but in general it is okay, the biggest problem however is that it lags or take too much time loading, per example when i click profile it takes up to 2~3 secs to load(sometimes takes a little longer), when i click match history it takes from 3 to 8 seconds to load, since i have 2 very diferent computers but the results are very similar i assume it is due to lack of optimization of the client OR at my internet conection (currently at 60~64ms ingame with a fiber option connection, no wireless used).
Another problem is the interface, the rune pages and masteries should rework to avoid too many extra clicks, it doesnt make user friendly like the old client, i dont want to click on a drag box and then select one set of runes/masteries, but thats just my opinion.
Big sugestion: I know you want to make a very shiny and neat client but i feel like its overdoing it, i feel it would feel nice to make some kind of semi-transparent or smooth background behind some features (Mostly behind Rune Inventory, Masteries, match History and Ranked), i attached a screenshot of the area i sugest to apply some kind of background, again this is mostly my honest opinion because since you can change your profile area to any champion wallpaper some of them doesnt fit so well with the color of the letters of the client