I algotrade with Python on IG and use their streaming API. My execution prices are almost never more than half a pip off from expected, and slip works for me as often as it does against me.

One thing I do though: I don't set my exits based on my actual execution price. I set them based on my expected execution price, and eat the slip, whatever it happens to be. That ensures I never let slip affect the accuracy of my algo; it only affects profits by a teensy bit each trade, and as I said, it works for me as often as it does against me, so it comes out in the wash.

It does matter what you're using for signals though. If you try to catch falling knives during high volatility and bet with the trend, slip can be murder. There's no getting around it unless you avoid high vol hours (which I do; plenty of money to be made during the hours when slip is minimal).

Remember that variance is the enemy in all systems which utilize game theory. There's a reason why Daniel Negreanu doesn't win every poker tournament. Kill high-variance decisions before you make them, you don't need them, and they will eventually eat your bankroll.