It sounds like your computer is swapping the contents of RAM to the hard disk---which is what happens when you don't have enough RAM allocated for After Effects. Show a screenshot of your Memory & Multiprocessing preference settings.
Good Points - you are partly right and my settings are attached here at the end.
I fiddled with them for a while (and even now they are too much in favor of AE) because it looked like they were not changing much from that weird behavior. The fact is that I tried most values - from max to minimum for background processes -nothing changed much except the speed of the rendering. The stumbling and the wild accessing of the main drive continued no matter what after the 14 seconds mark of all HD renderings.
Seeing than no one can help here either, yesterday I tried changing everything else in the Preferences to see what makes any difference in AE CC 12.1 and get over with this behavior.
And 2 unexpected settings really made a difference for me:
1-the QuickTime Video Preview, unchecked, ended the inescapable stumbling at the 14 s mark. I don't know how, but it made a crucial difference.
2- at the Memory page I checked the Reduce Cache size when system is low on memory. It prevented the RAM preview from using all the ram and leave the mac processes starving and turning the machine sluggish.
The Ram preview now happens in an interesting fashion (I have Memory Cleaner, a benign app, really useful to show how much RAm you really have at a given moment on a Mac). The green line above the timeline now expands until RAM decreases to low numbers like ~500MB RAM left. The Preview processing then waits a matter of seconds until the numbers increase again beyond 1.5 GB and then continues to extend the line - and so on until it reaches the end of the work area. There is no crackling anymore (accessing system HD).
I am using only SSDs and auxiliary HD drives (for output and source files) and all Ram previews now are completely quiet.
So, I have found my solution so far. But there still is a question the AE designers might want to think:
If they know that AE will unload the RAM onto the system HD, why don't they insert an option in the Prefs to allow this unloading to happen onto any secondary drive - choosen by the user.
A secondary drive unload at least will not interfere with the system processes - turning the mac sluggish and add insult to injury.
My numbers below could look tight now, but the previews are really fast and I quit all secondary processes on my mac when AE is working.
> If they know that AE will unload the RAM onto the system HD, why don't they insert an option in the Prefs to allow this unloading to happen onto any secondary drive - choosen by the user.
This swapping is an operating system process, not an After Effects process.
Thank you Todd,
Are you reading minds? Can you see posts while they are still edited?
The Blog is really great - thanks for everything.