Driveline slack? Slight “jolt” when accelerating

That's the trouble: service departments don't like subtle.

Maybe phone them, talk to a service advisor about it, establish what warranty is left, outline your concerns. I've got a hunch they'll say monitor for a while.

Has it always been thus, or have you noticed a progressive increase in the lag? Hard to say?

 

I would take it in right away, just in case it's a CV joint failure. You don't want one of those breaking catastrophically. By taking it in you also get the problem recorded, even if they don't know how to fix it, so when it totally goes bad -after- the warranty expires you still have warranty. Because it was identified as a problem during warranty.

 

Any Toyota dealer should have no problem with the warranty. Some dealers are better than others, just to warn you. Forum members in your area can tell you who's good. Not good with the warranty, but good with finding the problem and fixing it properly.

 

It may be normal operation. From a stop, when you accelerate, the Prius starts moving using the electric motor. If you accelerate harder (or speed gets above ~20mph) the ICE (Internal Combustion Engine) starts & the electric motor has to change speed abruptly. This may be the gentle "bump" that you feel. The "transmission" in the Prius connects the 1) electric motor, 2) ICE, & 3) front tires together. They are connected in such a way as no one device can rotate without another device rotating. ( <--think about that statement...) There is no "neutral" where they are not all connected! When you are driving down the road at ~45mph or lower & back off the throttle, the ICE will most likely shut off. This means that the electric motor must (at the same time as the ICE stops) change its speed drastically to keep up with the tires that are constantly rotating. As you are slowing down, when traffic starts moving again & you accelerate, the electric motor will start to work, but when the ICE starts the electric motor has to make a sudden change in its speed. I suspect it is these slight bumps that you are feeling.
These "bumps" will not occur at speeds of 50mph or greater as the ICE generally runs 100% of the time at anything above ~45-48mph.

 

Well there is another item - regeneration. When the Prius is moving at ~10mph or greater & you let up completely on the accelerator the Prius goes into regeneration mode where it "acts" like a normal car with an automatic transmission but is actually recharging the battery.
I haven't really noticed what you are experiencing but I may feel it but ignore it, I'm not sure now.
I'm sure if you can reproduce this "slack" on demand, then a ride with a Toyota tech should clear things up.
I hope it is really nothing so you need no repairs

 

There is also the issue of the different driving modes: Normal, Power, & Eco. I drive in Eco 100% of the time.
You could try changing to another mode & see if it also has the same trouble.
The three different modes just remap the throttle actuation rate. WOT Wide Open Throttle is the same on all of them, as is idle, but in-between gets remapped to be more or less aggressive.