Non-blocking read too slow on file/serial stream

[Jesús]

dear all,

I am doing a 'read' operation on a non-blocking FILE stream attached to 
the serial port.
Every non-empty read (about 120 bytes) is performed in less than 200 us. 
However, if no data in buffer, read returns -1 but the operation takes 
1500 us or more in average. Too much for my application...

Any explanation?
Any workaround?

Best regards
Jesús
Spain