While not actively documented this is a longstanding problem that comes and goes. Sometimes it's not that bad and other times it's borderline unuseable.
It's gotten worse. There was a time when it only happened to me in Peregrine Island or Talos during peak hours. Now it happens in KR at 3 am.
I'm trying to remember exactly when this became a consistent problem....and I think I may have first noticed it when trying to search my arcs for "profanity." It possibly could be related to that ***** ***** ****** filter? Of course I wouldn't put it past them to break two things at the same time; this is AE after all. I also wouldn't hold my breath waiting for a fix.
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
Arc ID#431270 Until the End of the World
Ah, well. Good to know, even if not reassuring. Thanks. 
I guess there is the option of editing text files directly outside of the game, but having to unpublish/republish every single time is kind of a hassle, and without proper syntax documentation lacking a graphical user interface is a significant drawback.
Just another thought to anyone else having problems with framerate in the editor: how good is your machine? I believe Zamuel has mentioned using a lower-end machine and I'm playing on a relic from a bygone era when men were men and women had spiral perms, and while I have seen sporadic complaints about editor lag it hasn't reached the epic proportions I would expect if everyone was experiencing it as badly as I do.
Note that I did not have framerate issues at all for the first year or so after AE release, and only had them in busy zones until a few months ago. It's not just my crappy computer to blame. They did something.
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
Arc ID#431270 Until the End of the World
I was having borderline unusable issues with the Ae editor while running windows xp 64 on a ASUS M2N68-Am PLUS Motherboard. When I upgraded to windows 7 my issues were fixed.
A friend said maybe its cause windows xp cant be updated past direct x 9?
Just thought I'd pipe in with my experience, I'm not a PC guy so I don't know why my issues with the editor were fixed. I'm just glad they were.
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Help Agent Martin in: A Favor for Liberty
AE arc #381565
My 1st. Video: Agent Martin in the new tutorial zone.
My beastie is not entirely awful in terms of processing power, although it's getting on in years (it was absolute top-of-the-line... three or four years ago, I think? Dual Core 2.67 GHz, 2 gigs of RAM, anyways), but I've been hitting that lag as well, although thankfully not often.
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MA Arcs: #12285, "Small Fears", #106553, "Trollbane", #12669, "How to Survive a Robot Uprising"
I'm more or less in the same situation as TeChameleon. My machine was great when I bought it in 2007 or so - dual core 2.4ghz, 8800GTX, 4 gigs of ram. I am running on XP 32 bits.
I have performance issues in other new areas of the game ; praetorian maps and so on, but nothing as crippling as the lag I get in AE. Even the dreaded warehouse room (you know the one) doesn't seem to be as bad.
My experience is also the same as Eva's. I used to be able to edit AE arcs without a hiccup back when it was released.
Tagging my name on the coat-tails of the last two posters, I'm in that boat as well. I haven't seen it be any better or worse under 'circumstance X' in a while, it's just always bad. I just sort of sit back and take things slowly... not much else I can do, right?
I'm having problems in AE too. I cannot edit a single character without my game crashing.
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Trying AE for the first time since I21 I have noticed, first, that the interface text looks a bit neater and more convenient to navigate, and second (more relevant to this topic), that I didn't have the performance woes I used to.
I only dabbled into editing for five minutes - enough time for me to realise as a tier 4 premium I can't actually edit AE arcs, duh! Why did you let me go all the way through the process only to stop me right at the end, game? 
Anyway, I was wondering if other people noticed better performance with the AE, or same old as before.
I quickly skimmed through the guides list and couldn't find anything related to this. When I try to edit my AE arcs, I'm getting terrible performance. Clicking on the "edit" button alone locks up my screen for a minute or so, and when I finally access the editing screen, framerate is in the single digits, which makes using the UI alone painful.
It's nothing new, I've ran into this issue for months (years?), but I just never bothered much with AE. Just wondering how do people who have the same problem deal with this, or if there's some option to make it better. It seems the game is still rendering the whole "real" game despite the window being in fullscreen, which might be why performance is so poor, but regardless which zone I go in or how low my graphic settings are, framerate plummets as soon as I get in the editing screen.