Download the Opera Mail installer, launch it, click "options", set the "install path" to a folder on your desktop, set "install for" to "Standalone installation (USB)" and install.
Once that Opera Mail loads up, create a a new account to add your Gmail IMAP account to Opera Mail (do not import anything). Do not change any of the default settings.
If it works fine there, there's something wrong with your normal install. If it doesn't work fine there either, I'm not sure what it'd be, but it's most likely something outside of Opera like your net connection and or anti-virus email scanner or some proxy you're running through.
For Opera itself, there's not much you can tweak. But, you can close down Opera and edit incomingN.txt in the mail folder (See Menu -> help -> about Opera Mail" for the location) where N is the number for the account as seen in accounts.ini. There you can set Force Single Connection to 1.
In Opera Mail, you can tweak some settings on the "Incoming Tab" in the account's properties. There you can choose to not download attachments by default or you can choose low-bandwidth mode (which includes not downloading attachments by default), but I wouldn't recommended that as low-bandwidth mode affects syncing and not downloading attachments by default affects HTML messages.
You can also set the availability of offline messages. By default, Opera only syncs the headers of messages, which will be pretty fast. The body for a message is only fetched when you view the message. You can change that to "make all messages available offline" though to download all messages in full right away. It will take a while for Opera Mail to fetch everything, but then all messages will open instantly. It will take longer for new messages to come in that normal, but that's because they'll be downloaded in full right away.
Besides that, you can goto "Menu > mail -> IMAP folders" and unsubscribe from "All Mail". Or, you can just goto mail.google.com and uncheck "show in IMAP" for it. The reason for doing this is to avoid 2 copies (at the least) of each message having to be fetched (as that's the way GMAIL IMAP works). But, you really should leave the Gmail "All Mail" IMAP folder enabled and subscribed as it's used for archiving and used for Opera's views.
Make sure you don't set the mail check interval to less than 10 minutes and the Gmail servers might complain. Since Gmail IMAP and Opera support IDLE (on "All Mail" and "Inbox" when using the default 2 connections), you'll get mail right away anyway and you only need to set the check interval to 30 minutes to keep the IDLE connections from timing out.