I don't know what else to say, we've had countless threads about the new client so I'm not wasting many words here.
Every patch the client gets buggier, laggier, and uses more resources than before. It's legit getting into my top 3 heaviest programs to run and it's ridiculous, because it's a client and not even the actual game.
Quick tl;dr of its issues:
1) Insane resources usage, like 60% CPU and 100% HDD for every patch (even 50mb ones);
2) Takes forever to log-in and keeps refreshing the toolbar on the top for no reason at all;
3) It takes forever to reload it after a match, even if the "close client during the game" option is unchecked;
4) Sometimes the loot/shop don't load at all;
5) Missions don't update at all and often need you to restart to see progresses;
6) The "disable esports notification" never worked.
And so on. It's unacceptable that Riot hasn't fixed anything about it in 2 years, it's about time to optimize it and fix some major bugs that make the client terrible and far from user friendly.