Get used to it... Motion isn't about creating video, it's about creating effects. As such, it's not all that concerned about playing back anything in real time. Motion can choke the fastest Macs to < 1 fps.
Motion is a memory hog. To render the project for playback, type Command-R. Depending on what's involved, it can take several minutes to render. If you're still running this on the Mac mini with 8GB, Motion may not render much more than a few seconds of project depending on the canvas size of the project. A 4K project will stress Motion more than a 720 project will. It is, however, recommended that you create projects of equal or greater size than the intended target (HD projects should be 1080, not 720, etc...) [8GB should be able to handle 10 seconds of 1080... (I really can't remember)]
What can you do to free up a few FPS here and there:
• You can turn off layers that are "finished" or not required for preview by unchecking their Layer
• Temporarily turn off extraneous filters [most of them have to rasterize the frame in real time]
• Motion tries to update the preview buffer in real time. You can sometimes help it out by going to Mark > Preview > Clear RAM Preview and let it start over from scratch.
• Emitters choke Motion faster than anything: turn the birth rates down to < 30, the lower the better
Turn it back up when you Save.
The Life of objects will adversely affect playback/renderings as well. If objects "live forever",
the number of objects Motion must "draw" increases geometrically as time goes on.
(again - keep the birth rate low as well if you need longer life.)
(Replicators are similar but to a lesser extent, depends on the number of "points" set)
• There is a Preference you can switch that will help out a little:
Preferences > Time, uncheck: Limit playback speed to project frame rate
• Command-/ turns on and off "overlays" -- toggle them off to play back
• (If necessary) In the Render Menu:
Turn on: Dynamic
Set the Resolution to Half or Quarter
Turn off: Lighting and everything below it (unless you need the feature)
• Turn off Preview under View > Layers Columns (always -- what a waste)
• Motion tries to update the preview buffer in real time. You can sometimes help it out by going to Mark > Preview > Clear RAM Preview and let it start over from scratch.
• If you don't need 3D, selecting the option for 2D Fixed Resolution for parent groups will keep Motion from trying to render anything outside the bounds of the canvas. When Fixed Resolution is not selected, Motion will "draw" every object anywhere in "space". In the case of emitters, it will draw objects thousands of pixels off the edges of the canvas; the group *grows* to the bounds of the limits of these objects and the rendering time will drop precipitously.
Sometimes it is actually faster to export your project to ProRes 422LT than to Render it (in project). You can create a file that you overwrite every time. If you have an SSD drive (internal or external usb3/thunderbolt), export to that.
For the details you listed, try unchecking the Filter first and play the project. If you get close to 30 fps (or more if you unchecked the speed limit option) you'll know that's the bottleneck (almost all filters rasterize a frame). The point of rasterization is that in real time is that the full frame of the "image" must be created, whereas other objects, only the region they occupy are placed in the canvas.
I use Motion for illustration as well as for motion graphics. Some of my illustrations cannot be played, they have a frame rate of greater than 5 minutes per frame. It's just something you learn to live with. Hope you hang in there!