Second home and investment property loans are more complicated than primary residences. A lot of low cost on-line lenders are not set up to do these or can't do them efficiently. If your financial life is complicated, underwriting your file requires extra work that the on-line lenders won't want to do. If you are going to get into investment properties, it can be helpful to develop a relationship with someone locally that understands what you are doing.
For one investment property and a simple underwriting situation, have a look at the rates and terms offered by PenFed credit union on-line. Their loan product should be a good representation of what you can expect.