Faster way of searching a string in text files

I need to search for a string, roughly 13 characters, in a group of text files using C#. The number of text files is changing and can range between 100-1000. The size of the files can range between 1KB and 10MB.

I tried the naive way of opening each file, read it line by line and see if the string exists (using index.of), but this is too slow. I also tried using the Boyer-Moore algorithm, which did improve the timing, by 5 seconds, but still this feels slow.

Any idea on how to speed up the search?