It depends on the research being done, but animal research is typically the earlier stage of research. You can't just give an experimental drug to humans before knowing what it does in vitro and then in vivo in an animal (rat/mouse then primate). Once that is successful it may move on to clinical trials on humans. This explanation is very generalized but you can get the picture.
Yeah I'd say there are a LOT more things you can do to mice than to humans!
Also another part of human clinical research is going through the recruitment and consent process, which takes up additional time/resources/paperwork, etc.