i had something similiar to this, hopefully this will do the trick for you, not sure why it did it though,

i found that moving the data out of the original folder into another folder cured the problem, it was something to do with the indexing i think.

another instance was 1 corrupt folder slowing the draw rate down. move the folders around until you find the bad one. move the data out, delete the folder, re-create it and put the data back