That’s why it is very important to post as many details as possible. If we had known you were not using the latest version that would have been one of our first recommendations. 🙂
Now try this: Close the project. Rename the folder(s) with your source clips so as to “hide” them from PPro. Open the project and offline all the clips. Now online each clip individually and hopefully you will discover one that takes too long. Replace it.
Split screen is achieved by scaling and positioning clips such that each one does not fill the frame, thus making room for other clip(s) in the same frame.
You will also find links to many free tutorials in the PremiereProPedia that will quickly show you how things are done in Premiere Pro.