Inventory Module Imports Inefficient

Good afternoon,

We post Inventory Imports through the inventory module (Tools -> Import Issues), and its getting to the point where daily import postings will take 4+ hours a day just to post through. At this rate, we wont be able to post daily anymore. The .txt file we are importing is about 12,000 to 15,000  rows with 11 columns of data, being aroung 1mb in size for the import. Are we doing something wrong where the module won't make this process any quicker? Should we be posting a CSV file instead? We can't test something like this due to the timing. We can only work after hours too since posting through the inventory module will halt our purchasing department from doing anything within the system.

Thank you for any feedback.