Old-style custom panel

I noticed that if the panel was created in earlier versions of the application, then it loads very slowly, and it slows down the interface very much. Moreover, even if you remove all indicators from the panel and delete everything and leave only the black background, the panel will still load for a very long time, while the interface will wildly lag. I recreated a panel of 20 megabytes in size, in the new version again and everything flies the interface does not slow down.
I don’t know how much this bug is?
I understand that the heavier the picture, the more brakes.
Well, in general, let’s say to someone.