Oculus Screen Lag with Particle Effects

Hey all,

I am creating a small scene in Unity where the user has to walk through a virtual space to find emergency victims. The emergency has to be obvious and so we set on a possible fire within a room.

The problem is, however, that as soon as I allow multiple particle effects to be located near each other (the smoke has to build up in density) the Oculus starts to lag. The regular Monitor has no lag at all though, and the scene runs at a solid 80+ fps.

Has anyone encountered this issue before? or can anyone help me with a possible solution?

I have already reduces the amount of particle systems from 7 to 3, and massively reduced the amount of particles they spawn.

The enclosed image shows the area where the smoke is located. The lag is most apparent in the door opening and when walking through the rightmost room.
The selected particle effect is WhiteSmoke2 (right bottom Inspector).
http://i.imgur.com/MVGaCtR.png

Thank you all very very much in advance.

My machine has:
i5-4440 / 3.10Ghz Quad Core
Radeon HD7900
8GB Ram
Windows 8.1
Unity 5.2.0 (cannot update to the latest version as the required Demos on the computer wont run with Runtime 7)