String split, substring is too slow

 Hi,
I need to process long strings about 2000 characters each. I need to break those strings into array of strings based on array of indexes.
 For example:   

"abcd123abc"
array of indexes {0,4,7}
final result:
abcd
123
abc
 Currently I am using regular String.Substring method, which is really slow, because internally Substring does a lot of range verifications.
Question: 
Is there a way to use internal wstcpy method to break my strings or may be there is some other  better method?
Performance is really important for me, since I am processing millions of strings.

 
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