I don't see how confirmations come into it. To arbitrage between exchanges, you own some bitcoins and dollars (or other national currency) in both exchanges. Then when an arbitrage opportunity arrives, you sell on the high one and buy on the low one. If long term your accounts are unbalanced, you can withdraw/deposit, but speed isn't required for that.
People weren't doing that because they weren't capitalized or there were barriers to becoming capitalized in the other country. Like there was no way to send national currency to the exchange in the other country because you couldn't get a bank account in the other country. But it was possible to send bitcoin to the exchange in the other country.
So people would create a routine circuit where they have dollars, buy bitcoin on US exchange. Send bitcoin to foreign exchange, sell bitcoin for that national currency (sometimes dollars). If possible, that exchange would allow wiring out, back to the user's real bank account. In other cases that exchange would have a bitcoin or national currency market of another cryptocurrency, like litecoin, where an arbitrage opportunity MIGHT still persist.
The litecoin would be moved and liquidated back on a US exchange.
Got to calculate it yourself and figure out why the arbitrage opportunity is still there. Usually there's a good reason. Sometimes there's a good reason that you are exempt from, and in those moments you borrow as much as a can get your hands on and make 5% profits as much as possible.
