The viewport update speed when rendering with infinite samples has been changed to work based on time since render start rather than sample count.
I have made this change because if you have a device fast enough to exceed 32 samples in 0.1 seconds, then the next viewport update you'll observe is 2 second after render start which can be annoying for anyone wishing to quickly iterate on fine detail parts of their scene.
Here is an example video of the issue (some of the "viewport changes" you see are due to compression artifacts to keep the video small):
{F10290321}
And here is my fix tested (the second demo with the caustics is there to show that the viewport updates reduce as time goes on. It's easier to see in the caustics scene due to the fireflies):
{F10290508}