STM32F4 :USB Mass Storage Write Low Speed Using FATFS Library
You cannot ever get to the "rated speed" on any peripheral link because the rates are usually expressed in raw bits per second, and every link has packet framing, encoding overheads, and protocol overheads on every level on connection stack. So the "1.5 MB/s" is not possible to achieve even in theory.

The internet-quoted transfer rates (800 kB/s WRITE and somewhat higher on READ) are achievable only on pretty sophisticated benchmarks, where the designers/coders took every precaution to provide the best possible data stream, using explicitly prefetched coding to reduce system overhead. If you are using some naked library and a regular sequence of I/O calls at your application level, you get what you got, 450-500 kB/s. Be happy with this.