While Opera is closed, you can copy ~/.opera/mail to ~/.opera/mail_new. Then, delete ~/.opera/mail and rename ~/.opera/mail_new to ~/.opera/mail.
Or, just move the mail folder to a different drive and then move it back. Or, use some defrag utility.
You can also open up opera:config in the address field and uncheck "Load Mail Databases Asynchronously" under the "Mail" section. That will make Opera load the database faster.
Also, you can check the database.
Besides that, the only way to clean omailbase.dat is to rename your mail folder, start Opera, add all your accounts back in and import your messages back into each account. You'll lose your labels and what messages are in them etc., but it's sometimes worth it. You also won't be able to restore feed messages or newsgroup messages. You can export your feed list and import it back in though and set up your newsgroup accounts again.
To import messages back into their accounts, use "Menu -> settings -> import and export -> import mail -> import generic mbox -> add folder" where the folder you want is in the store folder. Just make sure to set "import into" to the correct account.
Now, if you just want to try the mail folder rename and import trick, just launch opera with the -pd "path_to_new_folder" command-line switch to create a new profile. You can then set up the accounts in it and import from your regular mail folder. That way, you can test things out to see if it really helps. It must be the generic mbox importer for it to help at all though. The other importers won't help.
Maybe I can hide it somehow? Just keep recent emails and move others somewhere but I will have access to them if needed.
Press shift + F12, goto "buttons" and then "mail" or "Mail view". There's a period button/drop-down that you can add to the toolbar above the message list. This is a per-view setting that you can use to hide older messages.
However, messages will still be in Opera and in the database. If you want to move messages out of Opera, export them, create a new Opera profile and import them into it. Then, you can delete the originals from your normal Opera. Then, whenever you want to see those old messages, you just load the other Opera profile and look at them. Or, if you're going to do the rename mail folder and import trick, choose to only import messages from this year for example. Then, have another profile to import the rest of the messages for each account. It's up to you.
Opera shouldn't really be that slow. 90K messages isn't a lot for Opera.
