its fully depends on the function of plugin. For example there is a pluging to open ssh connection to an ESXi host. It wont create any load on vCenter or Host.
Where as if the plugin continuously query the vCenter for some information then there will be a slight impact on performance.
VMware doesnt do the plugin bench markings. If its a 3rd party plugin I would recommend to contact the vendor to check for their performance benchmarks.
