Plagiarism of Lecture Slides
If the author of the copied slides gave his/her permission for your professor to use the slides, we could reasonably have a debate about whether this is plagiarism. I would be on the side of those arguing that it is at least a mild  form of plagiarism if not worse, and in particular is setting a very bad example for the students attending the lectures, who are exactly the population that professors spend inordinate amounts of time and energy trying to educate about the wrongs of plagiarism and related forms of academic dishonesty. But as I said, I can understand if some people may disagree and argue that the culture of teaching in some places might look with some forgiveness on such practices.

On the other hand, if the author of the copied slides did not give permission to use the slides, then this is as clear-cut an example of plagiarism as I can imagine. To see this, try to imagine yourself as the author of the slides, who put in a large amount of time and creative energy creating something of value and will now be getting an email from the OP informing them that someone else has casually taken their creative output and put it to work for their personal gain, for free and without permission or attribution. How do you think you would feel? I do not need to imagine it; it so happens that a week ago I discovered that someone has plagiarized a work of mine. The details do not matter, and that incident involves plagiarism that cannot hurt me in any tangible way (though it is harmful to others), but I can assure you that it doesn't feel good. So to @ff524 and others advocating a tolerant attitude on this question, I hope you never find out how this feels from personal experience, and perhaps without this knowledge you cannot appreciate my argument. Nonetheless, I urge you to at least try the thought experiment I proposed above and reconsider your position.

EDIT: The premise of those arguing that this isn't plagiarism is that originality isn't expected in a teaching context. I'm afraid this argument doesn't stand up in the face of scrutiny. Let's say I were to show up to my class and instead of giving a lecture that I myself prepared, I were to recite word for word a lecture from a popular MIT OpenCourseWare class that I found on the web and transcribed, but without acknowledging the source or letting on that I was delivering someone else's lecture. If criticized, using the logic of @ff524's comments I will simply point out that teachers are in the habit of reusing each other's material, so it's no big deal. I will also claim that I am doing my students a favor since the author of the lecture is an extremely eloquent and charismatic lecturer whose lectures have been viewed millions of times by people from all over the world, so their lecture is obviously more compelling and useful than anything I can prepare myself.

Now, is there anyone here who seriously suggests that this can be acceptable behavior for a lecturer? Regardless of what you want to call it, there is serious dishonesty being committed. If I really believed reciting someone else's lecture serves the best interests of the students, why not acknowledge the source? My behavior shows a clear intent to deceive the students, to claim credit for the intellectual fruits of another person's labor, and to avoid criticism for not putting in the hard work involved in teaching a class, something for which I am paid. The situation described by the OP is 100% analogous to this, except my example is slightly more extreme. So again, before we discuss whether this is plagiarism I would like us all to agree that the OP's professor's behavior is 1. wrong, and 2. dishonest.

Now that we've agreed, the question is why anyone would be resistant to calling this behavior plagiarism. Okay, so maybe in a teaching context there is a bit less of an expectation of originality, but come on - copying 35 slides verbatim? Moreover, the dishonesty and intent to deceive that I described above are exactly the characteristic features of why people commit the more usual kind of plagiarism: they are lazy, don't want to work hard or aren't smart enough to do good work, and they want to get credit for doing something, so they choose the easy solution of taking someone else's work and passing it off as their own. Why the reluctance to call a spade a spade? I just don't get it.

Finally, let me elaborate on something I mentioned at the beginning. We in academia spend a huge amount of time and energy trying to inculcate in our students the moral ethos of doing original work and never acting dishonestly, especially in connection with claiming credit for someone else's work, and we agonize endlessly over why some students don't seem to be getting the message. I witnessed this on many occasions when I was serving recently on my university's Campus Judicial Board and sat on disciplinary hearing panels for students accused of many kinds of academic misconduct, including plagiarism. You cannot imagine the silly excuses and rationalizations people come up with for why they copied someone else's work or happened to be glancing directly towards the exam notebook of another student for extended periods of time during an exam, and you cannot imagine the frustration of the instructors who caught them. So, my point here is that if we refuse to label this professor's behavior as plagiarism, this makes us look like total hypocrites. If we are to have any hope of getting the message across to students that one must do original work and attribute any usage of someone else's, we have to be firm and consistent in our standards. I chose to focus on the scenario when permission for using the slides was not given by the author since there it is a bit clearer that a strict moral boundary had been crossed, but as I said my opinion applies also to the scenario where the author did give permission, although in that case it would be a milder offense since the author at least would not have been harmed.