Wrt ACS1900v2 wifi not competing with ISP modem

Hi Folks,
today we got fiber to our house and a new location of the isp modem and my router, the linksys wrt1900acs.
To my disappointment I find that my isp modem/router does a better job (speedtest.net) than my linksys...
The isp modem/router is fairly new so that might be why it does things better.

Location: ground floor in the middle of the house. Home office and living room is in floor above. We don't have to compete with other wireless networks around here.
I've done some testing on linksys, both on the 2.4 and 5.2ghz network but I'm not 100% sure of what I'm testing and if it will help. I've set the operating width to 40mhz on my 2.4ghz hoping to make it faster. But not sure it is faster with or without.

I've heard some talk about a different wifi driver, is that something I could try?
At the moment I have 12 devices connected to the 2.4ghz wireless, but only my apple tv, this pc and my phone is really active.
Is it so that if a device has really poor signal strength, that will be bad for the rest of the devices connected? Or is that only on 5.2ghz?

edit: linksys max at 21/22 up/down, my isp router 48/50 up/down. 500/500 fiber connection.