Ultra slow speed while transfer files between PC and Samsung S5
Wikipedia is our friend, again. :)

http://en.wikipedia.org/wiki/Media_Transfer_Protocol#Drawbacks

Quote:

MTP allows no parallelism, unlike USB mass storage or NAS. MTP has been built to only allow a single operation at a time (for example, read, write or delete operation), while no other operation can be executed until the previous operation is complete.

By design, MTP devices (like PTP devices) are not treated as a traditional removable drive. The actual file system is implemented by the device, and not by the computer's operating system. This also means that file system recovery tools on the computer will be of no use. In addition, most other programs on the computer will not recognise the MTP device, limiting the user to software from the device manufacturer or other MTP specific programs.

When opening a file, the user must wait until the file is copied to the host computer/device before it can be viewed; the file remains on the host computer's storage after viewing and after removal of the device.

Devices that are limited to MTP transfers do not allow or support instant access or multitasking.

The MTP and PTP standards do not support direct modification of objects. Modified objects must be copied out of the device and reuploaded in their entirety, which can take a long time for large objects. Google's MTP implementation in Android includes extensions to deal with this limitation; however, these extensions must be supported by the host operating system, and are not available by default. This blocks the direct creation of files on the device from the host computer; files must be first created on the host computer and then copied over.

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Most MTP-compatible devices are not assigned drive letters; instead, they appear as "devices" in applications such as Windows Explorer.