HSERIN/HSEROUT won't make any difference on it's/their own.

You need to look at the system level. Why is this code taking up to 300ms to execute? Because it sits here waiting for data for 300ms Before continuing (if no data arrives). So, the next question then is what is the F88 doing and how often does it actually SEND data?

And again, think about implementing some handshaking or polling scheme.

/Henrik.