A word on Add-ons
I'm a long-time Main Healing Priest, who only recently got into using Healbot. When I first started using it, in some ways I got worse. Partly due to interface lag, but mostly because I lost my focus on the Main Tank.
Of late, I continue to use Healbot as a secondary way to heal. I'll place the Healbot window next to the party window, or in the center of the screen surrounded by raid windows. I'll then use my right hand & Healbot only for occasional raid heals, dispels, and to give me a guide as to when to AOE Heal. That's it. Otherwise, I'm using the standard party/raid interface to switch healing targets, freeing my left hand to do the serious healing work on my primary target.
This 'two-fisted' approach seems to be working better for me. It did take me a couple of weeks of play to get comfortable with it, but I'm healing much better overall for it. Yes, Healbot has delays. That's ok. The trick is to learn to overcome them, if the benefit is there. For me it is.
"If the Truth can be told, so as to be Understood, it Will be Believed." - Terrence McKenna