I am using Outlook with Gmail (ISP uses Gmail for hosting). So, there are no rules applied to the inbox from Exchange, etc. In addition, I have compacted the PSTs. Let's forget about the network PSTs for a minute, as I have taken that out of the equation. Is there any reason why a 500MB main PST file (Outlook.pst) would cause Outlook to open so slow?
500MB is still a large file. When it is initially opened there is quite a bit of indexing going on which will have a negative impact on performance.