Uniswap (a completely decentralized on-chain exchange) process over $1 Billion in volume per day, that $1M is a tiny drop in the bucket. You can load that BTC on Ethereum as WBTC, swap for ETH, then swap for DAI/USDC and withdraw via about 50 different offramps. None of these trades will give even 0.01% slippage and total cost will be about $50 on Ethereum, or 10c on Polygon.