If You mean http request then its all that you can get.
You can not check anything after than requested server returns result for You, this is not up to You.
Exact speed will vary from link to link obvious. If Speed is too low then You just need to add more servers and run with more threads. That's how you increase speed.
I do not think there are such fast proxies.
To be the honest max I have seen was 10 Mbps, usually its around 1-2 Mbps for private proxies with a cost of 2$ per proxy per month.
So count yourself if that would be 1Gbps then it would cost You a fortune.
Again what I said before, You are adding more servers if speed is not enough. This is how it's done.