This might be because they are large sprites/images.
Perhaps if they can be split up somehow, like tiles.
Also if your panels are single large images you could try splitting them into tiles, it might reduce the load so effects can be used more, this is good practice anyway.
I finally noticed slow downs in some long stages in Doom mod (that's using same resolution), splitting the panel image up fixed the problem.
I'd always planned to do that anyway, but was leaving it until last once the stages and panels were final.