What model of camcorder is your video coming from and what format and resolution is it?
When you started your Premiere Elements 10 project, which project settings did you select?
Since version 11, Premiere Elements automatically sets up its project to match your video specs. In version 10, this had to be done manually -- and if it was done incorrectly it would cause performance issues.
Also, please do note if this software worked for you before on an early OSX and, if so, if it was on the same computer or a different computer.
I use a variety of cameras including a Nikon DSLR D750 and a JVC AVCHD HD handy cam. I have not used any settings other than whatever the defaults are. To be honest, I do not know how to change the performance settings. My computer (a mac) had the hard drive upgraded this year along with ram added and I needed to change to El Capitan after the upgrade; however, I was having similar issues before just not as bad as now. I still have the original discs for Adobe Premier elements (NEVER post a serial nr. moderator)
Mixing footage from that DSLR and that AVCHD cam could cause the program to choke in version 10 -- especially if your project isn't set up correctly for one or other other formats.
As I said, since version 11, the program is much more versatile and sets up its projects automatically. You can also mix sources much more easily. It might be worth considering an upgrade, even if only to version 11 (which should be available relatively cheaply). You'll be amazed how much better the program performs since its major overhaul in version 11.