First, The Mail Archive has a never ending capacity battle. More and more people use the service, slowing down performance. We periodicly upgrade software and hardware, improving performance. Second, under high load the service gracefully switches over to a more efficient batch mode, which means that many messages are processed at a time, not necessarily in the order received. Historically, the update interval for individual list archives has ranged from every few hours to several days.
Performance depends greatly on message traffic patterns.
Once a message is archived onto a web page, it will generally be indexed for searching within a week. Yes, we retain redundant copies of all messages. Only the most recent 3000 messages are available via date or thread indexes. Everything should be accessable via the search engine. Currently image attachments and excessively large attachments of any sort are unlikely to be archived.