Particle Object interpolation

I’m working with a keyed particle system with several key objects. The particles move slowly from one object to the next to the next. I want the interpolation of the particles between each object to be linear, but instead their speed ‘eases’ in and out, so they all slow down just before they reach each key object.

I know that you can change the interpolation mode between keyframes in the graph editor, but what about key objects? They don’t show up in the graph editor, so is there a way to make the particles travel at linear speed?