We are currently investigating and working on the performance issues. To help us figure out what is exactly happening we need players to submit the type of performance issue they're experiencing (generally low FPS, unexplained dips, etc) and their system specs and driver versions. The best place to send us that information is here, https://feedback.wizards.com/forums/918667-mtg-arena-bugs

Performance issues are tricky when we get a mix of feedback saying it either got better or worse, but we're not given any context on the types of PCs being used or what the exact issues are.