Well, your comment showing code is marked 'Deleted, but I can still see it, and I can tell you right now that you should be using a StringBuilder to build strings on the fly, only converting the StringBuilder to 'string when necessary.
Suggest you move the code example to your original post, and then format it with CP's code editor.
Also: look on NuGet, searching for 'Excel: [
^]: I see twenty libraries, many of which, I suspect, could provide code for file reading and mapping to Excel in ways similar to what you are doing.
I am also left wondering ... looking at the code sample ... if you could be using XML here for direct very-fast conversion ... but, perhaps, you have no control over the file format(s) you work with (?).