Chris Webb does a bit of sleuthing:
The Switch function is often a cause of performance problems in DAX. Last year a number of optimisations were made that fixed the most common issues (and as a result you should ignore older blog posts that you may find on this subject) but some scenarios still remain where performance can be bad; Marco and Alberto wrote a great post recently outlining some of them that I encourage you to read. How do you even know whether your measure is slow because of your use of the Switch function though? Trying to read query plans, as Marco and Alberto do in their post, isn’t usually practical and commenting out branches in a Switch to see if there is a performance change can be very time-consuming. Luckily, the new EvaluateAndLog DAX function can help you diagnose Switch-related performance issues.
Click through to see how.