Main reason for this load time is the fact every single Monitor relevant to that server is loaded and shown. In OM12 the behavior of Health Explorer is changed. Now only the unhealthy Monitors will be shown, like this:
This saves a lot of loading time thus making Health Explorer far more snappier. Another thing is that now only the relevant Monitors will be shown enabling one to zoom in much quicker to the issue at hand. So all the clutter/noise is removed.