Since passed in slideSpeed is checked through
slideSpeed || speed (ionic.js:8074,8075,8082) it means that a zero ms delay is replaced with the default transition timing.
I see no other method that transitions to a slide without an animation applied, so I guess this is unintentional? Applying a 1 ms delay sometimes causes a flicker.
This could be fixed by removing all
slideSpeed || speed and replace them with an initial
slideSpeed = (typeof slideSpeed === "number") ? slideSpeed : speed;. Not sure if this breaks anything else, though.