All the motion templates on VFXMarket.com don't use any plugins, they are made entirely just using Motion's built in features. Some of the templates are heavier than others and especially if they use a lot of 3D text, particles and motion blur. In these cases it is normal that they won't playback in realtime in FCP X. The suggested way to work with these is to turn off automatic rendering in the FCP X preferences and then render that part of the timeline manually once parameters are set. Select the generator and press "Ctrl-R" to manually render a clip.
Especially some gallery templates may play slowly if you use more than 3 or 4 high resolution videos in the image wells. You can also set you preview to "better performance" instead of "better quality". See screen shot. (Note this is only for preview and doesn't effect the quality of your final render).
If you are not able to make a template work for you, please do send us a message so we can see if we can make any changes to improve performance.