Unfortunately, you can't restore certain files from a .bkf file from a tape backup (unless there is a way that I am unaware of). You'll have to restore the whole 120GB .bkf to disk and then restore files from that.
Personally, backing up .bkf files is not the best practice. Instead of backing up folders with .bkf files, what you should do is create duplicate-to-tape job, in that way you can restore direclty from tape. Duplicate jobs can be configured either through a policy or from the Job Setup tab, right click on an empty space and choose "New Duplicate..." For more information go to the Help menu on BE, click on Topic and on the Index tab type in Duplicate Jobs.
On the slow restore, I'm sorry, I'm not sure why it's restoring at such a slow rate. I hope the duplicate job helps though.