Question Laptop lagging while resources seem fine

Dec 4, 2023
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Hello everyone,
I own an Asus TUF gaming 15 inch from 2020.
it has 16gb ram , gtx 1660 Ti and Ryzen 7 3750H with Radeon Vega Mobile Gfx 2.30 GHz.
I am using an external monitor (4k 27 inch).
when I play youtube videos on 4k the laptop lags alot both in the video and on the operating system generally.
When I look at the task manager I see the CPU is at 100% but the gpu is at like 4%.. which is weird cuz isnt the gpu resposible for these tasks?
attached is an example for the task manager while playing a video: