Correction:
I misspoke (mistyped) in my post above.
It should have said:
"The Airport Express will receive each packet, and retransmit it to the Time Capsule."
When used as a wireless repeater, the AE (or any WiFi repeater) sits in the middle of the wireless connection, receiving the packets transmitted from the base Access Point and retransmitting them on to the WiFi clients that are associated with the repeater (i.e. those in the extended coverage area). And vice versa for packets coming from the remote clients and sending them to the central Time Capsule. That results in significantly reduced throughput.
Another thing you might try is to connect another AP (your D-link, or another brand) to the Time Capsule with the longest ethernet cable you can manage: 20 feet, 50 feet, etc, and position the second AP closer to the area you wish to cover. WiFi propagation can be sensitive to the location of the unit, due to absorption or reflection of the radio waves. Sometimes, moving an AP a few feet one way or another can have a significant impact on the radiation pattern, depending on the environment (wood, steel, concrete, etc). You might get lucky and obtain better coverage from the second AP. Make sure the second AP is operating on a different WiFi channel from the Time Capsule.
Some APs have external antenna connectors, and can be enhanced with more 'powerful' antennae. In fact, they are not more "powerful" but are more directional and focus more of the RF energy in a horizontal direction.