How’s that different from leverage on forex trading?
Try asking $100M for a short time from any bank, I'll see how successful you are.
Last time I checked defi usdt lending rates seemed to hover around 8-9%. Not that exciting when you consider insane gas fees and all the hassle managing the contracts.
Nanoseconds of computational time.
If the trade isn't possible then the transaction fails and it doesn't go through at all. You pay the transaction fee.