Hi,
I'm having exactly the same issue. I've read through the above, and I tried using the -force-glcore argument in the launch options in Steam, but that made it no better. I've uploaded .rtf files of the player.log files to Dropbox, the link should be here: https://www.dropbox.com/sh/xxoy7rui5mpq753/AADjmREEFUCbPinoYO1WPzqxa?dl=0
I was running a couple of mods, but I removed them from the Game Data folder after the issue started, and logs above are without the mods. Today was the first time I've tried playing KSP since upgrading to High Sierra. I've tried two other games, both on Steam, and neither have any stuttering problems (Football Manager 17 and Euro Truck Simulator 2). The stuttering is so bad on both the first and second menu screens that the game is just unplayable once you get to KSC and beyond.
My game version is 1.3.0.1804. I'm running a 2017, 13 inch MacBook Pro with Touch Bar, 3.1Ghz i5, 16GB RAM, 256GB SSD, Intel Iris Plus Graphics 650 1536 MB.
I really hope this is fixed soon as I've finally got a bit of free time and was looking forward to getting a bit further on KSP.
I hope I've included all the right information!
Ps: I don't know if anyone else has come across this, but I tried to use pastebin (following the bug report link above) and the log files were too big for pastebin without a "pro" account, so I had to use Dropbox and upload them that way. Just FYI if anyone else tries to do that.