IIF vs CASE in SQL SERVER 2012

I've been told that behind the scenes, SQL Server 2012 converts IIF to CASE expressions. I didn't think that there wouldn't be that much of a performance issue. What I found in one of my queries is that if I ran the IIF first then the CASE statement. Each time I ran the IIF statement it ranged from 40 - 60 in the client processing time; meanwhile, the CASE expression would run in 10 - 30.

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