I too have this issue, and all auto sync options are turned off here too. The issue started a few weeks ago when I upgraded to 6.15.3.something, and continuing to 6.15.4.7934, which EN says is the latest as of this moment. In my case, the high CPU usage depends on the size of the note displayed in the note pane. If the note is small, a few lines of text or just one visible page of stuff, the CPU usage is very low and would have never attracted my attention. But on notes that are very large, say tens of MB in size and many vertical scrolling pages long, if displayed in the pane they cause EN to continuously consume 20-30% CPU on my i7-8550U CPU & 16GB RAM laptop. The fans get loud. As a workaround, I put a shortcut to a tiny note in that upper row where favorites can go. I click that note whenever I'm not using EN for a specific task.
I started a support ticket on this a couple weeks ago, after once again laboring to find the entry point to the well-hidden support process. I got a reply quickly, asking me the routine drag-me-through-the-mud "support" steps (like try reinstalling, send activity log). I started with this, but failed to answer an email within 48 hours so my ticket was closed. (That's a great way to make support even more unpleasant, and reduce the number of support requests received.) I have not yet returned to this issue with EN support.
Anyway, I notice that on a second PC, a desktop with an i7-7700T CPU & 16GB RAM that's running the same EN version, there is no increase in CPU usage when a large note is viewed. So this problem is reserved for my laptop, with its particular hardware configuration.