Would connecting an Airport Express to a Time Capsule slow down network speeds?

wifi networks are limited by the speed of the slowest member of the network. so if you set your network to be “802.11b/g/n” and only use computers and routers that use the “n” spec, then everything runs at “n” speeds. if you log on with a computer using “g” then the whole network drops down to “g” speed.

so in answer: YES. airport express runs no faster than 802.11g, while the time capsule can run 802.11n. to avoid slowdowns, use “n” only gear.