I'm no pro so they're likely pretty crap. I also learned that the 5D doesn't focus so well sometimes
Your "old" 6D does have a surprising benefit: ability to focus in lower light than a 5D3/5Ds/1Dx (and probably the rest of the line...I don't know how the spec measures up across the line). However, in sufficient light for the system to operate, I wouldn't call it a speed demon, but it's a well-proven system and I suspect there's a combination of settings that conspired to make your life less focused.
We're a Canon/Profoto family: five cameras, sixteen lenses, fifteen Profoto lights, too many modifiers.
I couldn't work out if it was the camera or lens, or maybe a setting I had wrong. In the viewfinder it looked sharp but on the computer screen it was a different story altogether.
"it was a different story"