To completely remove the stuttering and hitching during the new Soma Reactor OoB, you're going to need to reroute the game's log files to one that doesn't exist, since disabling it doesn't work.

1. Open Steam, right-click on Valley, click on "Properties"
2. Click on "Set launch options", then add "-logFile <PATH>" where you replace <PATH> with a file path that doesn't exist in a folder that also doesn't exist (like D:\not\a\folder\out.txt), without quotation marks
3. That's it. Enjoy a lag-free reactor OoB. Optional:
4. If you want to make extra sure, you can go ahead and delete the "output_log.txt" file from the Valley_Data folder as well.