It has been verified now. If speed info is modified from the detailed info of a drawbridge (speed info button), only the speed limit of that particular drawbridge is modified and nowhere else. If you go to the speedlists in the detailed list panel and select either speed list block or speed list simple and modify a particular speed limit there, all objects using that speed list block or speed list simple will have their speed limit changed. As said before, these lists are shared, which saves memory and load time, and also gives you the option to change the speed limits of more than one object at once if these object share the same list. Of course you can change them individually one by one which could be tedious, but even then you could setup a template from one switch or block, and paste the template data to other objects as well.