Isn't this par for the course when using an /assist macro?

First you have to wait for your comp to send the new target on your main, then the slave'srequest for the /assist target's target to the server, then it has to come back, etc etc - so your delay should be 1.5 times (about) your latency.

This has always been the case with my group. I usually switch targets when something is around 10-20% depending, that way they switch just about right.

Fear and targetting dead mobs can really mess with /assist - just esc and clear out your target and reaquire - not much to do about that.