So I spent a while longer banging my head on the wall, and I finally found a solution to my issue. @DaveNewton provided several links that lead to other links, and I found my way to here where some not-so-common knowledge is explained. It appears that for some reason the FILE_WRITE function is slow, and takes a lot of time to execute.
This can be substituted ...
From your comments I think I understand what you are after. So, assuming you have read some string into a String object called input and you have an array in to populate you can:
Clear the in array to a preset state
Copy the string content into in
memset(in, 0, 8); // Erase the contents of in
input.toCharArray((char *)in, 8); // Copy up ...