Yes as Julien states on single lane back roads it does slow. Annoyingly so on some roads that have small sections rated way too low (in my opinion). Like those small sections where a 35 drops to a 20.
But AP is not intended for use on these type of streets yet so cannot blame Tesla for that.
Also, if on a highway and the highway changes name it will also slow the car. Happens to me everyday, one highway technically ends merges into another one but in practice you just stay in the left 3 lanes to go one highway and right 3 lanes for another so traffic does not slow.
When there is no traffic 75 I would say is a common AP setting I might use.
If in AP the car goes down to 55 here.
The speed limit I know is 55. Not positive if before the divide the speed limit is 55, 60 or 65.
I changed my settings so the relative speed limit is at +18 and that changed it to slow to 65 ; which is still not what I want but at least won't cause an accident. Without that adjustment, the sudden slow down to 55 was dangerous. Since I set to +18 though, should slow me down to 73.
But otherwise, normal highway and the road changes speed limits, no the car will not change
However if you go into your settings and change the auto pilot speed limit to absolute instead of relative and use that, then the speed should adjust each time the speed limit changes on the highway. I have not tried that since I don't want the car doing that without my control but something you may want to experiment with.
Anything temporarily like a construction zone it will not read the lower limit though so no matter what the driver is responsible for knowing the speed at least currently.