After activating the WPMU plugins and setting them up, I went back into the Oxygen builder to edit something and discovered some weird stuff within the builder. See attached. After deactivating plugins one at a time and reactivating, I discovered that it’s the Smush plugin. It also slowed the back end a ton, but didn’t slow down the site on the front end (according to actual use and Pingdom). For now I’ve deactivated it, and I don’t really mind since it’s just a small one page site, but I’m concerned about future sites I plan on building on Oxygen and hosting on WPMU. I’ve permitted access to WPMU support on this site so you can take a look. Thank you for your help with this!