What is the best way (cheap and fast) to swap b/w BTC and ETH onchain? I don't necessarily want to use a CEX.
Personally, I've always had the best experience with instant exchanges. eXch is usually my first thought, even though it's technically still a centralized exchange, so maybe not what you're looking for. Just so you know, if you were thinking of checking them out, they're unfortunately shutting down on May 1st. (you can read the announcement
here)
lol, what is this instant exchange? looks scammy. Also, no CEX's please
Well, you asked for cheap and fast...
And no, eXch is not a scam. They've been in operation for over ten years.