Prop Firm Spreads and Execution Quality - A Deeper Dive

Been trading with a few different prop firms over the past year, and something I've consistently noticed is the variance in effective spreads and execution quality across the board. It's not just the advertised spread, but what you actually get filled at, especially on volatile moves. Some firms seem to have noticeably wider average spreads on $EURUSD or $GBPUSD during active sessions compared to others, and slippage can be a real killer.

My question for those with more experience: how much due diligence do you put into evaluating a prop firm's actual trading environment beyond just their rules and payout structure? Are there specific instruments or times you've found glaring differences in execution? Also, how do you factor in the often opaque costs associated with these firms when comparing them? Some have explicit commissions, others bake it into wider spreads. Trying to get a clearer picture on what's 'normal' vs. what might be a red flag.