Focustarget and /assist work at the same speed. There will always be that 1 second maybe even 2 second delay for the target to udpate. I'd prefer to say 1 second but it feels like an eternity I know.
I have all of my characters using /focus instead of /assist, so if one dies I can switch easily by making another character the new main through focus.
Your main though does not need to utilize focus. Thus leaving it free to set as a primary target or secondary.
Other than that I target via tab targeting, clicking, pushing the I'm feeling lucky button (aka instant kill). It's always wonderful when someone flops and you never had the targeted due to /assist or /focus not updating fast enough or Chain Lightning arcing over and splashing them with wonderful colors.