Minimizing AME Render Time

I just rendered a 9 minute clip that took several hours. It is one long green screen take, keyed with Keylight.. Background is a PNG., with a vignette and slight Gaussian blur. One title layer overlay.

iMac late 2013 27"

32BG RAM

OSX 10.9.5

Media Encoder 2015.4

Pulling cam files from a USB 3 drive, writing to another USB 3 drive.

The cam original is H.264, 90 mbps, 1080p.

I opened the AE sequence in Media Encoder,

outputting to DNxHD 145. I have 20GB dedicated to Encoder,

and for rendering GPU Open CL is enabled. Not even sure

if that makes a difference here, just trying to provide

all the details.

Is this just the nature of the beast,

or is there a way to speed things up a bit?

Would precomping the 3 layers in the AE sequence help at all?