It's possible that playing a YouTube video in Chrome is causing your GPU to work harder, which could be causing the fps drops in your game. This is because the GPU has to handle both the game and the video playback simultaneously, which can be quite demanding.
By full-screening the game, you may be reducing the load on the GPU by reducing the size of the window and freeing up resources for the game to run more smoothly.
You could try a few things to improve your gaming experience while also watching videos:
Try using a different browser for video playback, such as Firefox or Edge, to see if this helps.
Try playing the video in a separate window from your game, and move the video window to a separate monitor if you have one, to reduce the strain on your GPU.
Lower the game graphics settings to reduce the load on your GPU, while still maintaining a playable frame rate.
Close any other applications or programs that may be running in the background and using up system resources, to free up more resources for your game.
I hope these suggestions help improve your gaming experience.