I want to buy and sell BTC with margin trading etc, whats an alternative to bitmex?

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There's Kraken, but it has terrible, glitchy trade engine.

Not anymore, they finally fixed it.

Are you sure? They made an announcement a week ago stating that in another week they would give an update on their efforts to fix the problems

https://blog.kraken.com/post/1413/update-on-system-performance-and-upgrade/

There's nothing I can find on the blog yet. The only thing I could find was this status update saying problems with deposits and withdrawals have been resolved.

https://status.kraken.com/incidents/4y0x5fmh4skk