Liquidity Providers and Execution Quality for EU Equities

Been diving deeper into execution quality lately, specifically around mid-cap European equities on the DAX and CAC40. Beyond just headline spreads, I'm curious if anyone has practical experience comparing LPs through different brokers or prop desks when it comes to slippage during larger block orders. It feels like the advertised spreads are one thing, but actual fill prices can diverge significantly, especially during volatile periods or for less liquid names. Are there any general observations or metrics you've found useful in assessing an LP's true depth and reliability for these markets?