I don't know for Cisco SLBs in particular, but I believe some SLBs watch how responsive servers are. They then balance based on response performance. If such a feature is supported, it might even better satisfy your requirement if you're thinking is busy CPU makes for slower response. Since CPUs vary in performance, just looking at CPU load could work against you. E.g. a 50% load on a very fast CPU might be better than a 25% load on a slower CPU.