Yep, this is normal delay. If you're only waiting on the GCD between targets, then that's probably hitting right on the edge of how fast the "Main Switched Targets" data makes it from your main to the server and down to your alt. On my PCs at home, I sit at about 9-11ms latency, so I'm only suffering about half a second delays for this. When I was on cable, it was about 1-1.5 seconds delay between switching targets. On "raid nights" for your server, instance lag will extend this to up to 2-3 seconds at times, so that may be another factor.

Short answer: nothing you can do about that. :P

[edit] To simplify things, if you're setting the focus on your alt to your main, you can change the macro to be:
Code:
/cast [target=focustarget] lifebloom
Though I doubt it'll make any difference with the change-targets lag you're seeing.