You don't indicate whether you are trying to extend using a wired Ethernet connection between the Time Capsule and Express.....or.....whether you are trying to use a wireless connection between the devices.
We will guess wireless, since that is where speed problems almost always are found.
If you are trying wireless, temporarily, power off the AirPort Express and run a few checks on your Internet connection speed at the same point where the AirPort Express is located.
What are your results?
Whatever they are, if you are using wireless, you need to understand that the Express can only extend the quality and speed that it receives......and you have just checked to see what that might be. The Express is a "repeater". It does not "boost" the connection speed.
There is usually some additional loss of speed when you try to extend using wireless as well.
A good location for the Express when you try to extend using wireless is a point that would be about half way between the Time Capsule and the general area where you need more wireless signal.
In other words, if you need a better wireless signal in a remote bedroom, locate the Express about half way between the Time Capsule and the bedroom.
If you can use a wired Ethernet connection between the Time Capsule and Express, there will be no signal loss at all through the Ethernet cable.