We don't know what quantity "several" instances involves, the number of cores in the processor and the speed, if there are other CPU-hungry plugs and the LP-64 tips it over the edge, what SONAR's performance meter is showing, whether there are virtual instruments being loaded into RAM and the computer is doing hard disk swaps...all we know is that removing the LP-64s solves the problem. Given that it's a plug-in that stresses out computers, it sure seems like the computer is being stressed beyond a level it can accommodate. 

If a plugin is CPU hungry, you can also bounces tracks and archive the originals to free up some resources. This also allows going back to the initial track if necessary.

mettelus
If a plugin is CPU hungry, you can also bounces tracks and archive the originals to free up some resources. This also allows going back to the initial track if necessary.

Yes, excellent suggestion! Some people are not aware you need to archive to disconnect from the CPU; when applying mute the track is still connected, so it can return instantly when unmuted.

I just did a test thing on my aging Win7 32bit laptop with 4 gig of RAM, which is woefully underspec'd compared to even a modest DAW these days, and I managed to get 10 instances of LP-64 running quite fine, with CPU headroom to spare, even at 5.8ms latency.
 
While I absolutely agree this isn't the EQ you should be using on a track by track basis, on a machine that's much higher spec'd than mine, a single instance of it shouldn't be bringing it down, let alone multiple instances like I was running.
 
I'd be looking at what else you have running in the project first to see if this is pushing it over the edge (very possible) but if the project is fairly modest, or barely hitting your CPU without the LP-64 on there, I'd be suspecting something else that's causing the issue that this plugin is exposing. Perhaps a re-scan might help? Or trying this in a fresh project - something could have gotten screwy in this particular one (I've had similar weirdness with the Channel Tools plugin in the past, for example).

Run the freeware LookinmyPC app.  Uncheck all the checkboxes except 'running processes'.  Make sure you don't have something hogging resources/cpu.  One of my laptops for no reason at all started getting sluggish, oddly after a MS update had completed, and found there was some Microsoft app caught in a loop constantly sucking up 30-40% of the CPU.  Googling the app name turned up it was some useless font handler, and I disabled it through services.msc.