I was recently watching Schoolhouse Rock with my son and the cartoon for "Nouns" came on. There is a rectangular portal that acts like a giant television that the animated girl watches and at one point jumps into.
On this watching I was reminded of a science-fiction movie I'd seen as a kid that had scientists doing something similar. I remember very little about the movie apart from them I think initially falling thru the portal, and then at the very end finding their way back to the original lab where they see themselves moving at very slow speed. They step thru the portal again and then the movie replays at very high speed showing quick clips.
If I can remember more about this film, I'll add it. Obviously, googling for "Science fiction film" "Portal" didn't really give me any helpful hints.