btw....
"0% fees" is the most misleading marketing gimmick in perp trading!!!
Think of an airport currency booth with a big "NO COMMISSION!" sign. They'll buy your dollars at 0.90 euros and sell them at 1.10. That gap is how they make money. No commission = technically true!
Perp DEXes work the same way.
Every market has an order book: prices where people will buy (bids) and prices where people will sell (asks). The gap between them = spread. When you place a market order, you don't get the mid price you see on the chart. Buying? You pay the lowest ask. Selling? You get the highest bid.
Example: BTC shows $100,000 on the chart. Best bid $99,990, best ask $100,010. You market-buy at $100,010. Sell one second later and you get $99,990. You just lost $20 and the price never moved. That was the spread, and you paid it on a "zero fee" exchange.
Compare two venues:
Venue A: 0% fees, $20 spread
Venue B: $2 fee, $2 spread
The "free" one costs 5x more.
And it gets worse with size. The best ask only covers a limited amount, so big orders eat through the book level by level, and your average fill price gets worse each step. That's slippage, and it depends on depth: how much liquidity sits near the mid. Thin books show tight spreads that vanish the moment you trade real size.
True cost of trading = fees + spread + slippage.