What is this UMDF thing?
UMDF enables certain classes of drivers (mostly USB-based or other high-latency protocol
buses), such as those for video cameras, MP3 players, cell phones, PDAs, and printers, to be
implemented as user-mode drivers.
— Source: Windows Internals
UMDF, also known as User-Mode Driver Framework, came with Windows Vista as to provide a separation for drivers to running in User mode and in Kernel mode. The important difference that this provides is that when you attack a device (like a USB stick) that it does not need privileges to the user but can just run in the user context, such that you don't need to accept an UAC prompt or the like when you attach a USB stick to your computer.
So, it could be seen as a layer between the OS and those drivers / devices.
A further overview can be seen at this URL, the most important part here is the UMDF reflector:
The reflector is a Microsoft Windows Driver Model (WDM) kernel-mode driver that permits an application and a driver host process to communicate. The reflector creates a separate device object for each device instance and handles Plug and Play (PnP) and power I/O requests associated with each device instance. All communication between the application and the driver host process happens through the reflector.
— Overview of the UMDF
Again, this serves as a layer, but this time between the drivers / devices and your applications.
So, why is it slow in my case?
I'll tell you at first that it isn't slow. It's just a layer that doesn't impose much overhead, but then you might wonder why you are still getting that warning. Let's
take a closer look draw a free-hand ellipsis...
Right! So, that means Internal Storage is what is causing the problem here. Well, this doesn't tell much on its own though, that's why you should check the event log for more details on what device exactly is causing this problem and troubleshooting what might be wrong with that device.