My kiln has a peep hole with the plug. The plug material is generally a high-fire ceramic that can be removed for venting or peering within. Yes, there probably is some heat loss even when it is plugging the hole, but it seems to be relatively minor.
I like the hole so I can check on the glass (although it only gives me part of the story) and I don’t create a larger heat loss as when I open the lid.
The plug does get hot and should only be removed when you’ve got on your heat-resistant gloves and eye protection.