Improving speed of word search algorithm
widht is misspelled; it should be width. :-)
Some generalities regarding naming convention:

Variables and functions should start with a lowercase letter:

// "camel case"
int camelCaseVariable;
void camelCaseFunction() {}




// "snake case"
int snake_case_variable;
void snake_case_function() {}


Custom types should start with a capital letter:

class Class;


Macros should be uppercase:

#define MACRO 0

Check()'s body can be simplified as such:

return Letter == Matrix; // already returns true or false


Its parameters should also be passed by constant reference since they're not being modified:

bool Check(string const&amp; Letter, string const&amp; Matrix) {}

You should make sure the input file is open before proceeding.  If not, terminate from main() by returning 1 or EXIT_FAILURE.
You could vastly improve readability by using more functions.  Putting everything in main() is not always a good idea, especially with a lot of nesting.  Define the functions to perform one important task, and call them in main() or wherever else as needed.  Function call overhead shouldn't hurt you here, making the readability aspect more important.

In its current state, more comments may help others follow the code.  That, in turn, could help flesh out any optimization issues.  However, this isn't a substitute for writing readable code.