Though it's a quite subjective question but I feel it necessary to share on this forum.

I have personally experienced that when I create a UDF (even if that is not complex) and use it into my SQL it drastically decrease the performance. But when I use SQL inbuild function they happen to work pretty faster. Conversion , logical & string functions are clear example of that.

So, my question is "Why SQL in build functions are faster than UDF"? and it would be an advantage if someone can guide me how can I judge/manipulate function cost either mathematically or logically.