The iPhone 6 has a smaller wifi antenna than a computer, so its the trade-off, more battery life or more performance.
My iPhone 5S can reach 100mbps on the speed test under optimal wifi conditions. (my router is only N300, so this is normal)
What are you doing on the phone that needs the maximum bandwidth?
To answer your main question- It is normal for a phone to speed test slower than the computer.
You probably won't notice a difference between 10mbps and 100mbps as the next bottleneck is due to the processing power.
So, browser extensions slow down the speed? Not that I can see.
It didn't reach 1gbps due to the demand on speed test server, not the browser extensions.