I'd caveat this slightly as "it depends"

For mathy stuff, 100% c# is going to be better. But if you need to round trip to the engine a lot getting stuff in and out of the dotnet heap can actually hurt performance. You also have to be _really_ careful because there are a lot of cases you generate accidental garbage (biggest one is when you use strings that are getting implicitly converted to StringNames every time you run a function, you can avoid this by pre-generating them as consts but I've run into a fair few people who never ran dotmemory or the like to see the issues).