My main spell casting macros consists of:

/cleartarget
/cast [target=focustarget] Chain Lightning

My "slaves" never target anything, they only use the "master's" focus target. This is a little of an advantage in pvp because players that have my slaves targeted, don't realize that I have them in my sights until it's too late.

I have a heal spell that goes like this:

/stopcasting
/cast [target=focustarget,help.nomod][target=focus,nomod] Lesser Healing Wave
/cast [target=player,mod:alt] Lesser healing Wave
/cast [target=focustarget,help,mod:shift][target=party1,mod:shift] Chain Heal

This is a modified version taken from the Wiki. I added /stopcasting because I hate waiting for a spell to complete first and I added party1 instead of a toon name. This mother of all heal casting macros will heal my main's target if he has a target and if he doesn't, they will heal my main. If I hold alt, everyone heals themselves. If I hold shift, they cast chain heal on my main's target if the target is friendly, if not, they cast chain heal on my main.

But as far as the topic is concerned, I think the "speed" difference between assist or focus is subjective. For example, I was shocked when I read above that someone had a good experience with /assist over /focus. My experiences have been the opposite.