Perhaps even more depending on how slow your fingers are. ;)
I was recently made aware after over a year of pressing buttons that your spells aren't actually cast until you let the key go. This means that when you go to cast a spell quick to interrupt something or stop someone from dying with a heal there is an extra delay in the time it takes for your finger to not only hit the button, but to lift back up as well.
I only found out about this after I stumbled across an add-on to fix it, SnowfallKeyPress. Of course I didn't believe it as first, but I tried it and sure enough my spells were cast when I let go so I installed the add-on and sure enough they cast right away.
After trying the add-on out in a few instances and raids it was immediately apparent to me that my timing was a little off from what I was used to. Most noticeably was when I was healing I was often healing the wrong person because my heal went off before I had switched targets. I had never noticed how good I had gotten at synchronizing my target switching with my heals. That passed pretty quickly once I got used to the new rhythm.
The second thing I noticed, which is actually beneficial this time, is that I find myself able to interrupt much more reliably. Short cast times or catching an interrupt just as it comes off of cooldown, much easier now. This is why I will be keeping this add-on around.
When I started looking into this it appears many popular add-ons provide similar functionality of their own; so if you use an action bar replacement add-on or something like clique then this add-on might not do anything that isn't already being done.