I have 100 slides but most are light weight with pictures. I am afraid it is too much because I can't get it under 1 hour in my rehearsal. I have noted that some slides take me more time to explain. Any suggestions other than reduce number of slides? Maybe use some kind of timer.
Why are you fixated on the number of slides? What matters is the story you tell, the important messages that people who listen to you will be able to take home after the presentation. Sure, you'd like to say more, but that's not how it works. Don't plan to talk faster, to run through your talk at lighting speed. If the content doesn't fit the time frame, drop some things. You may not want to hear it, but you have to.
Another hint, maybe too late for you, but which I find useful: there are two things which are very interesting to know about yourself as a speaker:
How fast do I usually go? I.e., for a typical presentation, how many slides per minute do I do? This allows you to get a rough idea of timing, for future presentations, even before rehearsing (or allow you to skip rehearsing at all, once you become good at it!).
In real circumstances, am I faster or slower than during rehearsal? This depends somewhat on circumstances (being tired, being excited, etc.) but it is still interesting info. For example, I know that I tend to digress more during real talks than rehearsals, which means when I time myself I know I have to leave some margin. On the other hand, some people talk faster on the stage.
When my students preparer for their first conference talk, I usually ask them to think of the above, and write down their timing. It is useful to prepare later talks, and also their PhD defense!
To add to already good answers and comments. A rule of thumb often quoted is "number of minutes in talk"/2 but this number has a huge standard deviation. So both much more and less can still be good. Clearly you need to make sure you get your message across. I have seen someone using up 30-40 slides in a presentation in five minutes but then it was a way to create a "manual animation". It was nevertheless very efficient and gave the desired result.
The problem is that you need to consider, how much detail you need to explain your main points. You also need to think about how much time you need to discuss each slide. It is easy to just make loads of bullet points and actually not tell anything. In this case you need to consider your audience. If the target of the talk is your fellows in the department then they are not as deeply involved as peers within your sub-discipline. If the audience requires more background you probably cannot use a vast number of slides because people will soon lose track. With a specialist audience who can understand the slides more intuitively, you may be fine.
Speed is also a factor. If you have to much material you need to speed up and you are more likely to not finish on time. You need to pace yourself and consider how your talk will be perceived. A rushed talk is never fun. The result may well be a presentation where you think there are gaps. But that is how it is. You need to make decisions about what must be included and what could be discarded. Sometimes you can add the discarded material (figures) to the end of the talk in case someone asks questions where they can be used for the answer. That usually comes across as being well prepared.
Reduction. Since you ask if how you can reduce the slides, I will suggest looking at each and every one and think: Is this necessary? I am particularly thinking about figures. Necessary in this case means if it tells something unique that is used to build towards the conclusion(s). A follow up question before deleting it is perhaps if it can be merged with another slide?
Finally, and as a side comment, I do not think that you are new to Beamer is at fault. Using Beamer/LaTeX is great and for me the fact that producing a slide is slower than in, say, PowerPoint is a bonus. It helps me from just generating too many slides since I have time to think about each and everyone as I go through the process.