I just import and let FCPX create proxies while it loads the file into the event. Takes more time up front like you said but editing then goes much smoother. We may try shooting our next vid in 1080 just to see if there is a real difference on YouTube. I suspect most people won't notice.editing 4k on a anything is tough without proxies! I admit my laptop is a couple years old now but it's the kind with an extra Nvidia GPU and I still start any project by making 720p proxies for everything and doing timeline playback at 1/4 resolution (from that) to make it all work faster.
Probably people who do this stuff for a living prioritize having legit fast workstations..