First suggestion is go to the help menu and select detect and repair.
Who is your ISP? It could be a problem with their servers - check their status pages for any outages & problems in your area.
Check your mailbox using webmail. Is there an e-mail there that has a large attachment? This could be the culprit. If so delete it. If the inbox is getting near it`s limit this could also explain why it`s so slow.