Math library against own library in C#

Math library against own library in C#

So we tried developing a math class in C# and we did. Comparing results
with the original math class for System.Math shows that we are always late
a little or a lot (trig methods particularly).
But the wonder comes when we are using basic methods like Absolut value
which does not contain loads of code apart from
if(value < 0) return -value;
else return value;
and still we are far behind.
I cannot make this abs method any smaller, using the ternary operator will
not help either I guess.
Is it because the System.Math would be written in C? Would it go faster if
we write it in native language, though it seems it won't change much I
read. Finally, could a dll work faster than a class and if so why and if
not ...well why too?
Thanks already.