EagleFiler’s performance scales much better with large numbers of e-mails than with large numbers of files. The hardware that you have makes a big difference. On my MacBook Pro, I have a library with 10,500 files, and it’s sluggish, but definitely usable, when I’m editing the library, and quick when browsing/searching it.
You may find that it is more responsive if you work with a folder selected in the source list, rather than the entire library. In general, having more records shown at once in the list means more overhead for sorting, etc., when the library changes. And, of course, you could use multiple smaller libraries rather than one big one.
Making EagleFiler faster is one of my top priorities now that I’ve fixed, I think, all of the bugs that people have reported. Please e-mail me and describe the particular operations that you find sluggish, so that I can be sure to investigate them.