2gig fiber, why does passthrough never get true bandwidth

You're sorta in uncharted territory here.  You must be in NC. 

As a test, what happens when you plug your computer into Port 1 on the BGW and run a speedtest?  At this point I'm not even sure if speedtest servers can push 2gig or more so maybe test using a big download like a Linux distribution file.

I have "only" a 1 gig connection and my Netgear router gets the normal 940/940 when used with IP Passthrough although I'm on the BGW210.

The at home smart app runs a speed test to your att gateway it gets 2513 down and up.  Logged into the gateway run Speedtest on it 2450.  ASUS xt8 , in pass through, Speedtest from the router 1450 to 1600   I never test with laptop, always right from the gateway or my router.  There is this on going discussion how you lose some bandwidth on pass through if on fiber.  ATT has never offered the reason why as I am not the first to notice it

Gotcha.  So all of the speedtests you run are running from your gateway or router, never from a computer. 

The app and gateway tests are the same process, a connection from the gateway to a local AT&T server and is not influenced by Internet issues outside of AT&T's network.  The speedtest from your ASUS is going whoknowswhere so it's not shocking that it could be slower.

So by hooking your computer to the BGW and running a speedtest to a close by server that's consistent, you'll get a baseline.  Then plug everything back in the way you had it and connect the laptop to the ASUS and run the test to the same server and let's see what happens.

BTW....what laptop do you have that has an Ethernet port that's faster than a gig?

I doubt my laptop does.  My ASUS has a 2.5 gig wan port I am passing through. Both tests are going to same att server to test speed.  

here is test from router followed at the gateway 

So you don't have any device even capable of going over 940Mbps but you're unhappy that your router is going 50% faster than any device you have?

Again, too many variables on the ASUS speedtest and until you can hook something you can control where it tests from and test connected to the gateway and then connected to the ASUS I'll move along.

How do you know the speed test is running to the same AT&T server from your router vs the gateway?  Does it appear in the drop down menu in the router?  Unless you know the IP address of the server you have no way of knowing where its going. 

I can find Performance tests on the ASUS xt8 showing moderate to good Wi-Fi performance, but it doesn't look like anyone ever measured its full throughput to the Internet.  I'm wondering if the rest of the box can keep up with that 2.5 Gbps Ethernet port.

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@JefferMC thought I had seen some posts from you in the past about how it seems common that on fiber lots of folks seem to be losing bandwidth on pass through

I have noticed quite a few of those reports in the past, but not as many lately.  And some had issues while others did not.  

I’d start looking at the ASUS settings. Look at QOS settings, dns settings, etc.