If your having slow downs you can bake (export) layers that have for example; particles, replicators or heavy lifting filters. Bring the export (ProRes 4444) back into Motion and turn off the layer(s) you just exported.
An i3 iMac may not cut it when it comes to Motion. Motion 5 not only uses ram and the CPU, it uses the GPU too. Adding ram can't hurt it's not a cure-all if the computer itself can't do the job.
Here is Motion performance test I just ran on my iMac:
http://www.ubermediahd.com/blog/?p=137
My computer:
iMac 27" mid-2011
3.4GHz Quad-Core Intel Core i7
16GB 1333MHz DDR3 SDRAM - 4x4GB
2TB Serial ATA Drive + 256GB Solid State Drive
AMD Radeon HD 6970M 2GB GDDR5
Motion 5; no ram preview 17 fps, ram preview 47 fps...
Motion 4; no ram preview 24 fps, ram preview 60 fps.