Airport Express as extension
This question is similar to this question on MacRumors. Try using AirPort Utility to check the speeds you are getting from the AirPort Express. Go to AirPort Utility -> Base Station -> Logs and Statistics -> Wireless Clients. Then, check the box called "Tx Rate." You'll see 130-270 for 802.11n, and &lt; 56 for 802.11g. This will tell you what type of connection you are getting on your extended networks--i.e., 802.11a/b/g/n/ac. If you are getting very slow speeds, you may not be getting the speed you are getting from the TimeCapsule. Also, check out this question on StackExchange for determining the performance of your wired network. 

Hope this helps.