Is IFI compatible with tempdb growth? Is IFI (Instant File Initialization) compatible with tempdb growth? Recently I needed to increase my tempdb by 40 Gb and it took 15+ minutes to complete. I verified that the user had already been added to Perform Value Maintenance Task and also verified that other user databases took seconds to complete.

I didn't find any documentation or link that would have in-depth IFI details with regard to TempDB. I really appreciate your suggestions.
Yes,

Instant File Initialization works on TempDB Data files.  As with any other database, it doesn't work on the log file.  See Aaron's Blog Post to see where he shows IFI working with TempDB via setup.