The Time Capsule is a locked up device running NetBSD. Many have tried to jail-break it over the years but without success. Any user of the time capsule is limited for now to what Apple had chosen to offer in terms of file transfer, which is SMB or AFP.
Based on my observation, I think the problem with afpsf-ng is time-related. It seems that the Time Capsule and afpfs driver get out of synchronization somehow, especially at high transfer speed. So in order to make the TC to concentrate on file transfers, I relinquished it from its network routing duties and made it to operate in bridged mode. I also limited the rsync bandwidth using --bwlimit operand. I then ran a test over WiFi and the process went smoothly up to 4MB/s. I'll test again with a wired link between the TC and the clearOS server and report the results here.