How to teach from someone else's material?

In addition to the slides, use an accompanying textbook to prepare the lecture. This will give you additional context material to fill the lectures, as well as small exercises you can go through at the end of each session.

Just having the slides (and accompanying syllabus with general idea of how much is covered per week and what textbooks are used) doesn't make a course effortless. But they do save an enormous amount of time, so I am never ungrateful if there are existing (well organized) slides I can use.

So, yes, you will probably need to spend at least the same amount of time as the lecture simply going through and 'tweaking' slides to your own personality or reminding yourself of student potential confusions/viewpoint and thinking how to best lead them through the the tricky patches before you give the lecture. If you just flash through slides, perhaps you are going too fast - but if it seems as if the students are getting it, wonderful for you. Having extra time for review or for concrete examples can be invaluable for some topics and actually the best use of lecture time in any case. (As a side note, I teach physics so I'm not exactly sure what might be analogous to unfolding baffling derivations for students in fields far from physical sciences). I use slides because I can cover more material in the same amount of time :-) and they are more legible than my handwriting on the board. But that also means I need to know when to slow down.

Having someone 'tell' you what they say during lecture, or chatting about potential 'non' lecture activities for the course, is about as time consuming as giving the lecture themselves, so I'm not surprised the professor is loathe to spend much time after handing things over. However, at least in US, it would be very unusual not to have a syllabus that was passed out the first day of class that gave indication of 'where we are going when'. (Some syllabi are pretty useless, but if good for actually showing the organization of the course) and if you have a 'textbook' and a good set of slides to start, things are about as good as it can get.

I find it hard to teach from others' slides, which are often very dense. You need to "own" the material to have confidence to authentically teach it. I'd start my own slide deck abd pull in slides as appropriate. Check what's actually going to be assessed and focus on that. It's also helpful to add more exercises for the students to do in class to help prompt good questions.