50% FPS Drop Multiple Gfx Cards

edited March 2014 in MAIN

i have a  question regarding the way Millumin handles multiple cards.I know this has been discussed before but i'm getting nearly 50 percent Fps drop.
If i run a video across multiple outputs from one gfx card i get a steady 60 FPS . If i send only one output to a second card the FPS drops to 30 and stays there even if no video is running. 
 If i then send syphon out to Madmapper and send Madmapper to multiple outs on multiple cards the framerate from Millumin stays at 60FPS and the systems global framerate stays high on all outputs. 
I have checked the Vram usage in Xcode OpenGL driver monitor and it's not maxed nor is the first GPU's cores and AGPM is also working fine.

I understand that all rendering is carried out on the first card and then passed across to the second card but would like to understand the 50% fps drop when only using Millumin to output to multi cards.

I can send 7680 x 1080 syphon across to Madmapper with no FPS drop but can only run Millumin at 1920x 1080 pixel space across 2 cards before the fps drop.
I have tested with Photo jpeg, run Millumin at 64 bit and have also tested with prores 4444, 422, 8 bit uncompressed and others and the outcome is the same.
I would be interested to know why this happens as it would be useful to run Millumin only for multi card outs on some jobs.

Many Thanks.


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