Onboarding speed and platform latency with prop firm brokers

Starting to look seriously into prop firm challenges, and one thing that's nagging at me is the actual brokerage infrastructure they use. Beyond the fancy marketing, how quickly do people find the KYB process moving? More importantly, for those trading faster instruments like $EURUSD or $US30, are you experiencing significant latency or order slippage through the typical prop firm's broker setup compared to a direct retail account with a top-tier broker? The spreads look competitive on paper, but if execution is lagging, that negates any perceived advantage, especially for scalp-oriented strategies. Just wondering if anyone's had an 'aha!' moment about the real operational cost of their platform choice.