If you are using for-each then sequential process is unavoidable.
I have 2 solutions for you,
1. If you have several types of files(*.xls,*.txt, *csv) then you may have that much number of Foreach containers and direct them to process a specific type
2. You can 3 or 4 folders say Part1, part2, part3... and tell your 3 different Foreach loops to look at a specific folder. Probably for this its better to separate files based on size into each of those folders.
I'll post other ideas if I could think of any.