Yes, I have the same issue on a brand new server Centos 6.4 running Zenoss 4.2.3.
zenoss restart or start will take a extreme long time. Server doesn't have any high load issue. Not only web interface, but running zendisc a device took forever and hung.
top - 20:34:42 up 23:42, 3 users, load average: 0.01, 0.03, 0.00
Tasks: 227 total, 1 running, 226 sleeping, 0 stopped, 0 zombie
Cpu(s): 1.4%us, 1.2%sy, 0.0%ni, 97.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 49414476k total, 3519828k used, 45894648k free, 152088k buffers
Swap: 24780792k total, 0k used, 24780792k free, 373572k cached