How much time should I spend to prepare to teach a class?
Question: What is a reasonable amount of time to spend preparing to teach a class?


That question cannot be answered in general. Many factors go into what is a "reasonable" amount of time to prepare.

As commenters have argued, the first time you teach a class necessarily takes significantly longer as bootstrapping the teaching material can be the most time-consuming aspect of teaching. 15 hours per week for a new 3-hour course seems on the long side to me as well, but if the preparation for the lab is time-consuming it's probably not excessive. Labs in general vary widely in terms of their time commitment, based on the nature of the lab and availability of TAs.

What I have found is that it pays to be pragmatic: when I am teaching a class for the first time, I generally avoid time-consuming experiments and I do not provide much material outside of lecture slides, book (chapters), and papers (that is, I do not prepare custom readers or much else that needs to be created from scratch aside from lecture slides). I also try to go for a lab model that is considerate of my own workload in the first year (i.e., I'd rather try to have a project that students can work on more freely rather than something that requires constant hand-holding, if possible). In the following years, I will step-by-step create additional material or change the labs. This also has the advantage that by then I know better what additional material students actually need.


  I feel like I'm spending too much time and energy preparing for classes and this is hurting my research productivity.


Well, I would lie if I said that teaching a course ever did not impact my research productivity. It is a fairly seizable chunk of additional work and mental load, after all. In general, this should be built into your tenure- or promotion requirements, as it is true for pretty much every university faculty member (i.e., when your, for instance, tenure requirements are drawn up, people don't plan for you working full-time on your research). Also, try to figure out whether this is a one-time happening (for instance because the course is new) or whether you will always spend 1/3 of your work time on teaching going forward.

If you feel your current teaching load will prevent you from reaching your next career stage, you should either attempt to reduce the time you commit (e.g., be more pragmatic about how you design the course or how much you prepare for lectures) or talk to your department manager about whether it's possible to reduce your teaching load (e.g., give you more TAs next time, etc.). I feel making a data-driven case ("this is how many hours I spent on task A, B, C, because of reasons X and Y, how can I improve this for next year?") works best here, especially if your teaching load does not look extraordinary on paper.