Speaking from my own experiences, sometimes system overload spikes come from very specific plugins or specific processes not easily seen. I would open up Logic's CPU meter, look at the plugins your using in realtime, did you freeze the tracks or just bounce them? Maybe try right clicking on the track header and enable freeze, I think this would work even better because all the plugins on those specific tracks will get frozen as well, freeing up more CPU?
I've often wished that the node function in Logic would work with newer machines, but obviously you can't have everything you want, there is always compromises with recording.