Calculated fields are translated into queries and pushed back to the underlying data source where they are resolved, so results would depend on said source. My gut tells me that REGEX will be faster, as a single condition needs to be evaluated instead of 4. You can test this, though, using the performance recorder: Record and Analyze Workbook Performance. I'd try both approaches and see how long each query takes.
Hope that helps!
Below is not Tableau specific but the statistics should be more or less like that only. (as far as backend is concerned )
You may have more options with REGEX but you may have to pay for the same. (not 100% sure though )
Also you can your test with Create a Performance Recording yourself and see which is the fastest query.
Thanks, Santiago and Ritesh.
I've used the performance recording and the regex is much slower. Only once I've replaced 7 contains statements with 1 regex was there any improvement in speed.