external object cache computing cost

while doing a research on object cache, I read

The potential performance gain from using a persistent Object Cache is limited to the hits that miss the Object Cache. Using the default non-persistent cache hit rate of 95%-98%, the potential for performance gains is for between 2% and 5% of the request. In addition, an external object cache will add some extra latency because it is an external application – so it will slow down the 95-98% that were served directly in PHP before. The net gain from added latency + performance gain from external Object Cache often becomes negative for E-commerce stores.

Do you have any benchmark study that shows the hit rate without using the external cache ? or how much compute intensive is adding and using the external object cache ?