In addition to what Tom said, be sure to state what version of MacOS and what version of FCPX. If you are unsure about the codec, play it in Quicktime Player and do CMD+I, then post that info.
State what kind of hard drive the media is on. A split screen presentation pulls multiple data streams concurrently. A fully-rendered timeline is backed by ProRes 422 cache files. Multiple streams is a lot of I/O data per second.
If it doesn't happen with proxy files, this could indicate the system is just overloaded. But it should generally play OK using original media (not proxy) if the timeline is rendered -- assuming no I/O problem. Turn off background rendering, delete all render files with Files>Delete Generated Library Files>Delete Render Files>All, then do a one-time render of the timeline with CMD+A to select and CTRL+R to render. Make sure the viewer is set to "Better Performance". After that completes try to play it.
If it stiill won't play properly run Blackmagic Disk Speed Test on the drive holding the media and also the library and cache files.