[Excel 2010] Performance issues scanning external documents for text

Hey guys,
i need ur help in order to solve a problem with my code.
The goal is, starting from a compiled table in excel, to scan a single column and find, for each cell value, a match in the documents contained in a specific folder.
The code i wrote works but i have encountered many problems with the performance of the execution.
The code:

• Read the entire 3rd column and populate a collection
• When the excel reading is done it pass to the file management opening each one and searching it for each keyword previously stored in the collection
• When done with files search the control pass again to the excel in order to update an adiacent column with the search results

Now about the numbers: i have to search around 8-10 thousand values in about 800-1.000 word files.
The execution time, in seconds, is around

Formula:

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, for the specific case we're talking of 4 days of runtime, lol
Any suggestion about what to use (structures, search methods, etc..) to improve timewise?

Here the code:

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