I am currently having tracking issues with my CEM25EC and via my supplier have had communication from iOptron relating to guiding (which I cannot get to work at all on my setup - but that is another story). This is the reply from iOptron;
In general, well-made mounts with high precision encoders don't benefit from aggressive (rapid) guiding and some may actually suffer from it. Guiding with short exposures are not a good approach, mostly because the mount was probably just chasing seeing effects. Mounts with encoder often produce the best results with long-cadence guiding, meaning that guide corrections are issued on intervals of, say, 5 to 10 seconds.
The goal is to guide out the low-frequency errors that arise from polar alignment error, atmospheric refraction, flexure, etc. There's no need to use rapid guiding because the encoders should be keeping the mount on track.
This guiding cadence is established by using two parameters: 1) the guide camera exposure time and 2) the time lapse property.
If you use longer exposure times, you will dampen the effect of seeing variations right away - a good starting point for you would be 2-3 seconds or so. Then you adjust the time lapse property to create the final delay between guide corrections: camera exposure time + time lapse value. If you find that the guide star drifts too much in declination, you can reduce the time lapse value or improve the polar alignment.
The time-lapse property is not implemented in ASIair PHD. I have raised this with ZWO Support over a month ago asking for the feature to be implemented but other than an acknowledgment I have not received any further updates.
My apologies if this is a 'red-herring' but I thought it may be of interest.