Particles behave weirdly on cycles, they make cycles viewport preview constantly refresh, and it doesn't pass from the first sample.
This just happens on todays build, yesterday build works fine.
(example: 2.79b release)
(example: 2.80, edbf15d3c044, blender2.8, 2018-11-28, as found on the splash screen)
Short description of error
Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).