Rendering Frames to JPEG get slower progressively

I always render stills frames from 4K with some noise reduction, lumetri and shadows tweaks in order to present the client the final look and feel, It tooks a lot of time to render the final comp, so this metod is the best way I've found to make a control before that.

I've found the first frames are rendered at 1 each minute, but it gets slower with time, the first 10 are 1 frame x 50 seconds and in the frame 20 is about 1 frame x 3 minutes, the frame number 20 is about 1 frame x 7 minutes!!!

I guess this is a RAM problem, BUT I can't understand why AE isn't freeing memory as if I close and open again the program it starts again with the best speed possible, and it gets again worse in a few frames....

Sadly this is what I have o do, as they are around 300 frames, the fastest way is to set and alarm so I can close the program and open again every 18-22 frames to get the best frame rate again.

I'm not an expert in AE, so maybe I'm doing a lot of things wrong here.

Thanks

(BTW, I use he script "render frames at markers")

Macbook Pro 15"

i7 2,6Ghz

16GB RAM

GeForce GT 650M 1 GB