They’re not ints in C++11 
UENUM is just a macro for UE reflection system, it does not magically make enums heavy on performance.
TEnumAsByte is just a wrapper, that allows to display enum inside editor, if you’re not going to expose enums to the editor, you don’t want to use UENUM and TEnumAsByte.
However, access to reflect data might induce some performance hit, since it’s another layer on top of the code, in other words, it would be faster without reflection, AFAIK.
Hope this helps somehow.