Partitioned Database restore is very slow on SQL Server 2012 with SP2
Well I don't wonder about the restoring time. I wonder about your file amount. 15k files for 112MB? What the h...?

Your symptom is nearly the same as during file copying. If you have two disc (disc1, disc2) and you try to copy 100.000 files each 4KB in size from disc1 to disc2 it may take hours. If you copy one zip file without compression (containing those files) with 400.000KB file size and then extract it afterwards on disc2, it would take a few seconds.

You'll stressing your I/O subsystem with many I/O operations. Consider to merge those files back to one or just a few. I'm not sure what has driven you to those design?