Just read that the Vray group found OsX 10.11 to be slower then OsX 10.10 due to:

Well, we do have a performance issue when drawing our buckets in the VFB, that is Mac OS X specific.
The problem seems Qt related as it doesn't happen in MODO 701/801 where V-Ray for MODO uses the wxWidgets library and not Qt.
The problem seems to have appeared in Mac OS X 10.10, and has become much worse (like 10 times slower) on Mac OS X 10.11, with no change by us.
It basically adds a fixed overhead to each rendered bucket, so the smaller they are, the more time is added to the render.
This issue is not present on Windows. You could check if this is the reason for the slow-down, by making sure that the VFB is completely below other windows while rendering.
Haven't investigated why it happens exactly.

Greetings,
Vladimir Nedev

http://community.thefoundry.co.uk/discussion/post.aspx?f=34&t=119375&p=1000890 [community.thefoundry.co.uk]

So testing Houdini it appears to be a very similar thing.

OsX 10.11
IPR render with ‘Preview’ checked:
2 min 39sec

IPR render without ‘Preview’ checked:
50 sec

OsX 10.10
IPR render with ‘Preview’ checked:
1 min 19sec

IPR render without ‘Preview’ checked:
43 sec


NB: Can't submit bug as the webform is missing the Captch. OsX 10.11 Chrome and Safari

Vladimir (V-Ray for Modo developer) has said that the bug has been resolved in the latest nightly builds. Also Modo is using a newer build of QT than Houdini and the bug may only be related to those new builds.

I'm one of the Modo beta testers and have mainly been gearing up for the 10.0 release next week. But once that's out in the wild, I plan to check the new V-Ray nightly build for Modo to see if the problem is indeed fixed.