Rounding ints division result to certain "granularity"
Let me start with the usual:

Naming


myDecimal: Usually, the prefix "my" does not bear any sensible information. You should think about the information you want to pass by the types name. (For example Decimal is often the name for decimal fractions and the way in which numbers are stored in it!)
rest: in the context of a modulus this is called remainder
candidat: this is a candidat but where is the other one and which one is it? You should call this something like: flooredValue


Reusability

Everytime that you use this function you have to use the exact type or must cast, which is slow. You should think about making it a template function.

Performance

Most of this is micro optimizations but you seem to be interested in every last bit of time. I don't know how much of this is automatically done by your optimizing compiler:


go to assembler: This is very easy code that can be written in assembler. Especially the division and modulus are two functions that are slow (compared to others) and (at least) on x86 architectures the assembler instruction for division gives you the modulus at the same time. (likely optimized by the compiler)
multiplication by 2: this can be replaced by a left shift by one (very likely optimized by the compiler)
multiplication in return: instead of using a multiplication you can use an addition to return the results (return val - remainder; and return val + granularity - remainder;) (maybe not optimized by the compiler)


Note that all of those "optimizations" could be done automatically by the compiler and even more important might not be optimizations on modern architectures anymore. When it comes to performance you should compare on the assembler level and do extensive profiling.