To reproduce:
1. Open the attached project;
2. Open "Bug" scene;
3. Make sure Virtual Reality is enabled (Edit > Project Settings > Player > XR Settings);
4. Enter Play mode;
5. Find the ParticleSystem.Update in the Profiler

Expected result: ParticleSystem.Update takes ~0.00ms.
Actual result: ParticleSystem.Update takes multiple ms and produces low performance when VR is enabled.

Reproduced on versions: 2017.3.0f3, 2017.3.0p1, 2018.1.0a7.
Not reproduced on versions: 2017.2.1p1, 2017.3.0f2.

By design: We cannot simulate the particle system until the previous frame is finished drawing it (otherwise we risk modifying data that the render thread is still using).

VERIFIED Fixed 2018.3.0b5