I had the same problem and also I notice that with the toggle preview set to false the situation is not improved, and investigating is because the rendering of the camera is done anyway.
The slowdown is due to the PostEffect Rendering on the camera GUI preview.
I've made a couple of changes moving the camera render only if the toggle is to true and added the option to enable or disable each post effect for the preview.
Attached there are the modified files, hope this can help!:)
[EDIT] I tried to attach the files but they seems to don't appear in the post, what should I do? Thx