In my experience… I would keep using Multisite for your setup for organizational purposes and ease of management.
Here’s a list of things you could try before emptying out the pockets:
- Consider caching options. (Personally, I like using
W3TC), but there’s
many options for this. - Consider Jetpack CDN for images. (I’ve had reasonable success with this.)
- Consider tuning your Web Server, Database Server, and PHP
Configuration according to your system specifications to get ideal
performance. - Consider upgrading to PHP 7 (if not already on it) — It’s significantly faster than PHP 5.
- Consider PHP-FPM?
- Consider Hyper-DB?
One of the biggest overlooked options of course is code and query optimization.
I could be wrong with my ideology here, but I would assume that if the server only had to load one set of files it would result in better performance versus loading two sets of files.
If it’s clear as day that one site is completely hogging resources, and you’ve made every effort to optimize your setup. THEN perhaps at that point, it’s time to move that site to it’s own server.
Also, before you immediately jump to a dedicated machine, consider all the optimizations I mentioned above and beef your VPS resources. It might be cheaper to double your VPS resources than full blown dedicated machine.