Fidelity now paying 7.25% (down from 8.00%) to borrow TSLA.
@durkie - when I was evaluating brokers, my decision came down pretty quick to IB and Fidelity. I can't explain the disparity between the borrow rates you're seeing, and the rate you're actually being paid. The way I read IB's literature, there is some small amount of the rate paid that goes to overhead (transaction costs). My impression is that it's a really small amount - 0.1-0.5% kind of scale. Close enough that I mentally rounded it to 0 when I was evaluating IB vs. Fidelity.
My primary suggestion to your comment is to give IB a call and ask them about that. You may also want to ask about the rates people pay to borrow shares to establish a short position - does their rate change on a daily basis, does it stay the same as when they borrow (unlikely), or how does that work.
The opaqueness also seems to be what I see with Fidelity. Then again, others have reported circumstances where their shares weren't lent out, the brokerage was reporting 0 shares available, and more than a day went by. Then they called and asked, didn't get much of an answer, but there shares were lent back out the same day. So just making contact with IB about your question may also change your outcome.
From what I can tell, Fidelity sets the two rates separately, with sometimes paying >50% of the rate they charge to borrow, and sometimes paying <50%. Overall, it seems to work out to 50-55%.
I mostly chose Fidelity because IB is clear about being a brokerage for frequent traders, and I'm not. The characteristics that make IB friendly for frequent traders are either irrelevant or a negative for me.