TimeMachine: FileVault2 to encrypted external HD unendingly slow

I have following set-up:

MBP (the newest) with FileVault encrypted Flash-HD

1) TimeMachine on TimeCapsule, not encrypted, works well.

2) TimeMachine, on external HD, USB, encrypted: takes unendingly long.

(17GB of 260GB in 8h)

Process «mtmfs» takes ca. 70% process power during this time.

The HD is ok - I can read and write things to the encrypted Volume with no problems, including reading the backups.