Onboarding speed and slippage differences across prop firms?

Been diving into a few different prop firm challenges recently, and one thing that's really jumped out at me is the massive discrepancy in both the onboarding process and the execution quality. I'm talking about getting verified in minutes vs. days, and then seeing significant slippage on $EURUSD trades with one firm compared to almost none with another, even during similar market conditions. Is this just down to their underlying broker choice, or are there other factors at play, perhaps related to their specific liquidity providers or how they manage order flow? Curious if others have noticed this too, and if there are any specific firm characteristics that tend to correlate with smoother execution and faster KYB.