It's a common issue: above 300Mbps often other things become the bottleneck rather than the broadband connection - either wifi or the client device CPU or the application software.

If you're doing a real large download (not a speedtest) then it won't be doing the browser animations so your speed won't be affected.

And even if your CPU or hard drive were the limiting factor in a download, it would just leave bandwidth spare for other devices on your network to use.