If you're running an arbitrage bot and looking for brokers with slightly slower execution or more predictable behavior, there aren’t many options left — most are too optimized now. That said, you might want to look into Axi and OANDA.
With Axi, especially through their Select program, you get access to real capital and now both MT4 and MT5 are available. Execution is fast, but depending on time of day and pair, you might still find moments of slippage you can work with — just don’t expect it to last forever.
OANDA is another option — solid reputation, regulated, and known for stable infrastructure. Their feed isn't always the fastest, and some latency traders have said it’s “workable” depending on setup.
Either way, this kind of strategy doesn’t last long on one broker — best to rotate, keep size small, and don’t get too comfortable with any single setup.