In Applications Manager, Websphere Application monitoring is done via PerfServlet.ear for version 8 and below. For version 9, monitoring is done via PerfMBean ( via JMX ).
The PerfServlet is not designed to run concurrently. Being a single threaded servlet, it would collect the data sequentially from available servers. This single threaded operation can cause higher response times when the PerfServlet is used in larger deployments.
If you are facing performance overhead in your Websphere servers ( version 7 or 8), you can edit the monitor, make the version as 9 and update. Monitoring will use PerfMbean instead of PerfServlet.
PerfMBean feature available from Websphere Application Server (WAS) version 7