RT Times spike when over 60 plots pass filter - Why?

Whenever more than 60 plots pass the filter, our RT times go up exponentially. It appears our farm has a threshold which I don’t understand fully. Strikes me as a possible performance issue I’d like to understand so we can enhance our network and address.
Will the community enlighten me on why this would be the case, if it should be concern when RT times spike up sharply to 2000ms?