I am trying to download the game on the PC but the download speed is very slow.?

The first fix you can try is pausing the download for a few seconds, then resuming it. Since you're downloading from the launcher, your progress will not be reset even if you do this multiple times. If that doesn't work, you can try exiting the launcher from the taskbar and relaunching it. Restarting the PC may help but not entirely necessary.

https://www.appgamer.com/genshin-impact/strategy-guide/how-to-fix-genshin-impacts-slow-download-in-the-launcher