Doesn’t matter how fast the connection between them is if the disks or devices you’re reading from (and the one you’re writing to) are the bottleneck…
For example, the theoretical max throughput of a SATA 3* controller is 6.0Gb/s, so even if you have such a controller at both ends, under ideal circucmstances with a drive at each end capable of maxing out said controllers you’re still some way off. throw in some overheads, real-world conditions, ‘normal’ drives and an OS not optimised for throwing files around and you’re going to go SIGNIFICANTLY slower.
Not to mention the difference between transferring one big file and writing a single stream Vs thousands of smaller ones and associated overheads.
*I don’t knwo what controller is in your devices but SATA 3 is a decent enough example.
EDIT – unless I’ve looked at the wrong device:
Apple Time Capsule specs
specs say the Time Capsule only has:
Gigabit Ethernet WAN/LAN ports…