TL&DR in the end.
I have always thought it was a problem with the game or the web UI. It never accured to me that this could be the issue, even though the same happened to me a couple years ago with the WoW PTR servers.
Going into nvidia 3d settings and choosing nvidia gtx 1060 for wow ptr had solved my problem before. However in this case I wasnt sure which exe had to choose, considering the web UI might be different than wc3.exe.
So I didnt even try that. I went to device manager(where you can update or disable drivers) and I disabled the Intel HD Graphics driver. Now the main menu is as smooth as butter.
Tip: Do not uninstall the driver, just disable.
TL&DR: Yes, disabling intel graphics worked for me. Thank you @Mongoose and @Caterpepl