I had a 4Gb Kingmax USB flash which was working instantly until I had to format it. After that the copying of 1.5 Gb file takes ages!
I tried to find the answer in Internet but vainly.
So I found the solution by myself.
Follow the instructions bellow:
You need to reformat your flash to FAT32.
ATTENTION: ALL DATA ON FLASH WILL BE LOST, PLEASE MAKE A RESERVE COPY BEFORE CONTINUE.
The standard "Format" tool in Explorer context menu does not allow to select the allocation unit size when formatting to FAT32.
So I will suggest to open "Computer management" from context menu of My Computer->Manage.
Then select Storage->Disk management and Format on Removable Disk you want to format.
Select file system for flash drive FAT32. In a drop down menu for "Allocation unit size" select maximal possible value that allowed for FAT32 it should be 64K or 32K. Ensure that in "Format Options" you DID NOT selected "Quick Format".
Select "Start" to run formatting process.
This is working solution, at least for my flash drive.