Smaller companies are often quicker at responding than large companies. I think that this is because there are fewer layers in choosing who gets interviewed, their need for someone is more urgent, and they are less likely to have rigid procedures for their hiring process. There are some small companies that are exceptions though. It could be because they want to do a thorough job of hiring the best candidate or because their leaders are very indecisive.
For large companies, there are often rigid procedures for hiring. This varies by company. I don't know if you can make judgements about their hiring process on whether or not they are a good company in general. At many large companies, the human resources is very separate from everything else.
With agency recruiters, my experience is that some of them are very pushy. They want you to push you to agree to submit your resume and to agree to a phone interview ASAP. Sometimes that is because their client wants a candidate right away, but often they are pushing their client to fast track their hiring process as well. Others are more cautious because they don't want to risk presenting an applicant to their client who might not be a perfect fit.
Ideally, a company or recruiter will let you know when you are out of the running. Some people recommend contacting the potential employer if you haven't heard after a certain amount of time. I usually don't do that unless I have had an interview. If I have had an interview, I usually wait a couple of weeks.
The only times I have been told why I wasn't hired was when I was a finalist. One time, I was the local plant's top candidate, but was rejected by corporate because they thought that I was "immature". The other time, I was one of two finalists and the other had more product specific experience than I did. The other times, I received the standard form letter that said that they hired someone that was a better fit. I think that some of not letting candidates know is a matter of time. Other times it is because the hiring manager simply liked one candidate more than another. In other cases, they may have already have someone in mind before the interviews and don't want to admit that.
I agree that some people have an advantage in getting hired for good jobs. It often isn't a fair contest. I don't think public agencies, schools or even some employment agencies do a good job of helping people reach their employment potential. Social mobility is limited by more than just educational attainment like politicians and schools pretend.