Slowing Render Speeds

So I'm having a strange issue that I've seen on a couple of Macs of late...

Render speeds start high then gradually slow down through the course of the render by a 1/4 or a 1/3.

Yesterdays example, I was rendering 12mins Arri Alexa Plus footage (ProRes 4444, 1920x1080) of lens charts with Rec 709 LUT:

Starts at 67-9fps and by the end on the first 1 min clip we're at 53-56fps.

At end of the next clip we're at 49-50fps. We stay here for a while.

By the last clip we're hovering in the 43-47fps.

I had a similar issue with a D700 nMP working with Arri Raw and I put that down to overheating. I find it frustrating as its gradual drop off and it just feel like I'm not getting the best out of the machine and the Nvidia based macs I've used in the past offer a more stable render speeds with older hardware. Anyone else experiencing this?

I'm running Davinci 11.3.1 Lite on a new MBP 15" (Mid 2015) in OSX 10.10.5

2.8GHz Intel Core i7
16GB Ram
AMD Radeon R9 M370X

I had a similar issue with a D700 nMP working with Arri Raw and I put that down to overheating. I find it frustrating as its gradual drop off and it just feel like I'm not getting the best out of the machine and the Nvidia based macs I've used in the past offer a more stable render speeds with older hardware. Anyone else experiencing this?

I just expect it as the way things are. I'm amazed you can get faster-than-real-time performance out of ProRes 444 HD files on a MacBook period.