How does one interpret Load Average in iStat menus?
Load average is CPU based, not GPU based. The numbers represent one, five, and fifteen minute averages of the load on the CPU.


  A single-core CPU is like a single lane of traffic. Imagine you are a bridge operator ... sometimes your bridge is so busy there are cars lined up to cross. You want to let folks know how traffic is moving on your bridge. A decent metric would be how many cars are waiting at a particular time. If no cars are waiting, incoming drivers know they can drive across right away. If cars are backed up, drivers know they're in for delays.


Source: http://blog.scoutapp.com/articles/2009/07/31/understanding-load-averages

1 core with a load of 1 means that core is 100% busy. But you can add loads when heaving more cores. 4 cpu's with a load of 1 means the total of the 4 cores is 25% busy, a load of 4 means 100%. A load higher than the number of cores means that the cores can not handle the load an the cpu is running 100% per core (and is the bottleneck of your system)