There are a few ways to fix the issue if you are affected.
League of Legends players enjoying the ongoing Star Guardian event, complete with a new in-game story and skins, may notice that it also brings an unintended side effect that can limit the power of their PC processors.
Multiple players across social media have pointed out that the animations on Leagues client pertaining to the Star Guardian event are lagging their PCs. While the animations in the Star Guardian event only involve subtle movements, they appear to be taking large percentages of CPU data, which can be viewed in the PCs task manager.
Such large data usage can lead to lag in other tasks that may be open, including accessing the internet and other games. Users on Reddit found that this lag can be attributed to the animation being rendered in above average quality automatically, and needs to be lowered if players want to continue to see the animations without lag. This can be done by heading to the task manager, going to the tab labeled details and lowering the correct task from above normal rendering to a lower option.
Switching off of the Star Guardian event tab in League seems to resolve this issue, as well as turning animations off using the option in the top right corner of the tab. Players that experience this lag may also want to head into their League settings and allow the option that closes the client while in-game.
Riot Games has not yet issued a statement on this issue that appears to be affecting a large portion of the League playerbase. The Star Guardian event is currently ongoing in League, League of Legends: Wild Rift, and Legends of Runeterra, and is expected to continue until August 15.