You can certainly use two hubs, though at the moment you'll have a hard time finding a second one (they're currently out of stock--hopefully more coming soon?). But what you suggest is indeed possible. I actually do almost exactly what you're suggesting: all my motion-lighting rules are on one hub (I want those to be as fast as possible), and the rest is on another. Technically the first hub also does all my Zigbee devices (I didn't want another mesh) and the latter all my Z-Wave (most of which aren't used directly for lighting), but I think my lighting hub is now free of all custom apps except ones I wrote myself, so it's more or less achieving my goal. 
However, the problem with your hub is one I'd be interesting in solving regardless. There's no good way to track things down without putting in some effort, however. Some things you can ponder:
- Did you notice a change after installing a new app/driver? Try disabling (see the "X" icon in the upper right of the device and app list to expose this option) or uninstalling it to see if that helps.
- Are you using any apps that are known to cause issues for some people? I hate to point any fingers here; older versions of webCoRE were problematic for many, but I think people have better luck with the current fork. The beta Chromecast integration seems to cause slowdowns for some. If you went wild installing custom apps and drivers, those are definitely things to consider (and a second hub dedicated entirely to questionable code could indeed help the other)
- If you're using all stock apps and drivers, the above would be less of a concern, but I'm sure staff would be interested in anything you find. If you have poorly written rules (one of the few built-in ways you can cause problems for yourself given the power it provides) or questions about those, feel free to ask! If you are using lots of custom apps/drivers, consider built-in solutions first--they can do more than some people give them credit for.
- Sometimes you can also look at logs (or really Past Logs) to figure things out: anything throwing a bunch of errors? That could be a poorly written app or driver, or it could point you towards another problem. But general slowdowns may not produce much of interest here.
Finally, you can also consider some odd cases that might make things worse:
- "phantom" Z-Wave devices (should be cleaned up automatically overnight and probably not an issue for many people in the first place, especially if you didn't have problems including a Z-Wave device and had to try multiple times)
- Dashboards with "use all devices" selected (makes them slower to load but I'm not sure it would have much of an effect beyond that) or "ghost" tiles that point to a since-removed device (something we've been told recently isn't good)
- probably more that others can add to this list that I can't remember at the moment
... but these are all less likely and probably not where I'd start my work.
Rather than doing much troubleshooting, some people also schedule a reboot if they find that a reboot is helpful (as it may be, at least temporarily). There are different ways to do this, including a few rules in this thread and some community apps you can find in others. I'd consider this a band-aid myself, and it's not something you should have to do, but if it works... 
