massive speed drop (90%) on Apple Time Capsule wifi network

The usual issue is that 5ghz is giving poor signal and so your devices are connected to 2.4ghz which is very limited speed.

Use the Mac internal wireless diagnostics or use a free analyser like netspot for Mac OS and check your wifi signal

In a Mac hold down the option key and click on the wifi fan in the top menu area.

Also for clients connected to the airports you can see from the airport side what speeds are involved.

hover your mouse pointer over the connection.

This gives you a lot more hard numbers which are essential to analyse the problem.

If you are linking to 2.4ghz you must change the name of the 5ghz so you can force the connection to the higher speed but perhaps lower signal level of 5ghz.

Then you need to get rid of extend airports.. sorry but they kill the speed of the clients. The old N wireless extreme and especially the express (toy router) will give you very poor speeds. In a normal extend wireless setup it halves the speed or worse over what would always be bad result due to age of the extenders.

You should be able to get full speed from the AC time capsule.. but it does also depend on range and age.. things get poorer as they get older.