Technically, it’s somewhat expected as Airtime is shared so one at a time, and if one of your client is slower, you have to wait for that one to finish.

So more using, more you lose the speed is a norm.

Now can you do better? The answer is yes. That’s why there are expensive routers with better QoS, or high density prosumer/enterprise access points to handle multiple connection better.

In short, you can’t get 1300 Mbps.

But in short, find a matching client for MIMO e.g. if 1300 Mbps is for 3x3 WiFi 5, you need that client e.g. MacBook Pro.

Then in close proximity, you can do iPerf or even Internet speed test (but this is variable so may need to check different time of day, different server).

The max you are looking at is the Max # x 0.5-0.7. It’s WiFi efficiency factor.

https://gameandtechfocus.com/wifi-college-why-is-my-wifi-slow-theoretical-max-speed/

Fast.com tends to give higher numbers. One of reason I personally don’t trust them (for accuracy). When I do iPerf (local test), Xfinity with correct server location seems be more concordant with my result.

However, relatively speaking I think they can still be used, just to absolute number.