This happens during workflow processing, not prior to (or upon) activation. I am the sole user of a virtual Dev environment developing a state machine workflow which uses custom Task Status content types that are unique to each state. The workflow is associated with a list, being activated upon item creation.
Starting from the workflow link belonging to the primary list's item (which takes me to that primary listitem's task/history page), I right-click and edit the initial task, changing the workflow to the next state. Upon saving the OOB edit form I am returned to the primary listitem's task/history page. It doesn't refresh, but instead - after a few seconds - displays the 'Heavy Load' note. I manually refresh the page and all is as it is expected to be: the first task is completed and the next task is queued.
If I start from the Task list and edit the task, the only difference is the lack of the 'Heavy Load' note, that is, the previous state of the Task list is redisplayed after saving my edit, and as soon as I manually refresh it all is good.
I have tried running the site from an account with only Contributor permissions, and have accessed it from another virtual machine on the same network; neither have made a difference in behavior. To my knowlege the timer jobs are all set to the defaults. The workflow association has also been moved to the primary list's content type (for a different reason) and this has also not affected the outcome. I have created a trivial state machine workflow (Initial, Processing, Complete), using the OOB task statuses that simply moves from Processing to Complete upon any change to the Processing task - same outcome.
The 'Heavy Load' note seems spurious since a) it is happening on a single user dev site and b) a simple page refresh is all that is required for me to be on my way. Does anyone know why this is happening, and more importantly, how to rectify it before putting this into a production environment?