problem of I/O in UMAT

Hi 

Has anyone met the problem that if  adding the I/O (just reading
externals or writing externals) in UMAT, the computing speed becomes quite
slow, but the CPU occupied by the ‘standard’ process is few, sometimes even keeps zero all
the time. The I/O is quite simple, just using the 'OPEN' , 'READ' and 'WRITE'.

 That would be great if anyone knows the reason for this problem.

 Thanks and Regards

Yinhua