the free feed vs paid feed decision is bugging me more than expected

I went through the Alpaca connection guide last night, mostly out of curiosity, and honestly the mechanical part is nothing. Make the key, paste two fields, done before my tea cooled. What I keep sitting with is the bit it tells you to read before you trade at all, the part about which market data feed you pick. The guide is pretty direct that this is the choice that actually decides whether the whole thing works for you, not the setup. And I believe it, but I'm not sure I understand it in a practical way for how I trade. My own style is slow. I hold for days, I don't scalp, I've spent years teaching myself to stop reacting to every tick. So my first instinct was that the free feed is obviously fine — I'm not fighting anyone for a fill. But then I thought about it more and I got uneasy for a different reason. If the prices I'm seeing are only part of the picture, then my entry alerts and my stop levels are being measured against something slightly different from what actually happened. Not a big deal on any one trade maybe. But I keep a journal, and a journal built on partial data is a journal that will quietly teach me the wrong lessons about myself. So the question I can't answer from the guide alone: for a slow, low-frequency approach, is the concern about the cheaper feed mainly about fills and slippage, or is it also about the honesty of your own backtest and review? Those feel like different problems to me. One costs you a bit at the point of execution. The other corrupts the thing you use to decide whether you're improving. Has anyone here run the same strategy on both and noticed the difference in their records, rather than in their P&L? That's the bit I'd genuinely like to hear about.