Slow Export as file

I'm trying to export a feaure film (90 mins 24bit 48khz 800M) in 5.1.

The stereo export of the same film takes about 1 minute 15 seconds, in both interleaved and multiple mono.
The 5.1 exports in REAL TIME ! 90 minutes to export ...??? ! (again in both multiple mono and interleaved)

It is NOT session specific. I started having this problem about 9 months ago but have found no other posts related to it.
Last month I updated to PT 10.3.10 (MAC 10.8.5) and hoped I would get the speed back .. but nooooo.

The workaround I use is:
Highlight Region
Locate highlighted region in cliplist (click the little triangle to see all tracks)
Click 'Reveal in finder'
Highlight in finder
Copy, Paste

What could the problem be ?

Merci for any info.