Thanks for everybody's replies!
@Corjan-Meijerink
However, speed limits often do not change in sharp corners. Suggested speed limits might but that does not always is a known factor in the road data. Warning about sharp turns is an interesting topic by itself. However, it the road is curvy / dangerous, I just look out for the road sings on the actual road
Not being arrogant but just honest.
I might have phrased that badly; I neither thought of warnings for sharp corners, nor do I want to suggest a digital surrogate for the actual speed limit signs 
I would prefer the app to remain silent and the rider to focus rather than warn for every upcoming obvious turn.
I 100% agree! I, too, don't want to be flooded with warnings or hints. Plus, I don't consider sharp corner warnings useful, personally, because I have to be cautious anyway: Is the road wet? Are there little stones or dirt patches? etc.
What I did mean, though, is a "upcoming speed change" hint as a respective speed limit symbol before you can see it in reality. Why is this useful?
- Example 1: German Autobahn, unrestricted, only two lanes. You're in the slower one on the right, going 130 km/h and reach a point where the vehicle in front is you drives only 100 km/h; on the left lane, people pass with 180+ km/h. So, is it worth to look for a slot to safely switch to the left lane and overtake? No, because in 800 meters there's going to be a new limit of 120 km/h, so overtaking now would just be an unnecessary risk and waste some fuel for accelerating and directly slowing down again.
- Example 2: Driving on a curvy country road, going 80, but 100 is allowed. A curve comes up and looks fun; but you know what, 75 meters after that curve a village begins (which I can't see, but thankfully the app shows this on the track as a little "50"-speed limit sign, so I let the bike "roll out" (slow down on is own) rather than accelerating.
So, overall, for me this would rather be a fuel saving and "knowing what to expect when I can't see yet" function. I don't think the app should warn me or anything, just zoom out a little and show the sign where the speed change will occur (the data is already there, anyway, it just needs to be visualized).
(By the way, is there a possibility to set a preferred maximum speed that gets taken into account when I drive somewhere by bike or car? With my motorcycle, I typically don't go much faster than 130 except for overtaking maybe, while with my car, I'd rather go 140 - 160, depending on the traffic situation. Had I a more luxurious car, maybe it's would be more like 160 - 200 or so. I never know what to make of calculated travel time for longer routes, because I don't know what underlaying presumptions are being considered by the app.)