Have you tried doing any arbitrage on any of the DEX out there (the only one I have experience with is the Stellar DEX).

If you manage the offers yourself (rather than going through some client) there are no exchange fees, and the transactions are nearly instant.

There could be two approaches to it:

1) Find pairs/paths within the order book that can make a profit. There's practically no fees here so I suspect the margins will be pretty thin.

2) Use "anchors" to convert your Stellar assets back into the real cryptocurrency, and then deposit again them via another anchor (or exchange) and complete your trade.