Generally speaking, if you want to view/search all the messages at once, it’s more efficient to have them in one mailbox. Multi-GB mailboxes are OK; EagleFiler doesn’t try to load the whole file into RAM, or anything. The downsides to having a large mailbox are that (a) you’ll waste time and RAM if you only want to access a subset of the mailbox, and (b) when you merge two or more mailboxes into a larger one, EagleFiler has to index it, and with a larger mailbox that will take longer.
Well, technically there’s no way to split up a mailbox once it’s in EagleFiler. You could stop merging after a point, though.
There wouldn’t be a measurable effect on the launch time. If you’re searching them all, it would be faster to not split the mailbox.
So is there a upper limit on this? It might be ok for the OS X and EF to handle, lets say 20GB mbox file, but if it means slowing down system too much then I think a ‘general rule of thumb’ on the upper limit would be good.
There’s no hard upper limit. Any given amount of data, accessed simultaneously, will use a certain amount of time and RAM, and those amounts will be somewhat less if the messages are spread across fewer mailboxes. Of course, large files can be unwieldy for other reasons, e.g. backups or if you ever want to open them in a RAM-based text editor.