When you start doing complicated documents in Illustrator, full of live effects and crazy magic, things start to slow down. Not surprising, really: you’re asking it to do a lot of work every time you need the screen redrawn. It makes some attempts to cache as much as it can, but it can only do so much. Here are some things you can do to help it out.
Change the preview mode. In the past few years, Adobe added the capability for Illustrator to use your computer’s graphics card to draw the preview. And if your work is beneath a certain complexity threshold, it’s great – it runs faster than the old CPU renderer ever could, fast enough that they’ve added a switch for “realtime editing” where everything you drag around updates at full color instead of as an outline. But if you cross that threshold? The performance drops off a cliff. Hard. And suddenly you are staring at the spinning rainbow cursor for multiple minutes when you’ve changed something, or just tried to move the view around. The old CPU renderer can still start to slow down in extreme examples of these cases, but it won’t be anywhere near as bad – I’ve never gone beyond “a few seconds”, even in my heaviest drawings.
What mostly seems to invoke this is (1) a lot of layered transparency and (2) a lot of bitmap effects. But I’ve managed to make it happen in other circumstances, like a recent piece that had tons of complex, opaque pattern fills. You can do a quick check to see if this is why things are slowing down by doing view>view using CPU; if Illustrator suddenly starts performing better, then you’ve found it. Note that “view using CPU” only affects the current window on the current document; if you routinely work in ways that make the GPU renderer choke, then consider going to the Performance panel of the main prefs and turning it off permanently.
Maybe the GPU renderer works better if you have a cutting-edge graphics card in your desktop system that can get 600fps in the latest games. I’ll never know, I haven’t owned a computer I can’t stick in my bag since I lost my G4 to Hurricane Katrina.
(This is also often the solution to “I have weird render glitches in Illustrator”, by the way. The GPU renderer is a lot newer than the CPU one, and GPU compatibility is a constantly moving target. Each major release of Illustrator since this was added has had distinctive GPU rendering bugs that pop up on some computers; usually when they get fixed, new ones get introduced. Maybe eventually the Illustrator team will figure out how to find them all and make sure no new ones pop up despite graphics card manufacturers constantly changing things out from under them, but maybe someday Sisyphus will get that rock to the top of that mountain, too.)
Layer thoughtfully. Divide the paths that make up your drawing into layers with meaningful names. And then turn off parts of the drawing that are finished, or are not something you need to see for what you’re working on now. This is super easy to do with comics: I make each panel as a set of layers inside a layer with a clipping mask shaped like the panel border, and while paying attention to the overall composition of the page is important, there’s also a lot of parts of the process when I’m zoomed in on one panel, and can turn off the rest to gain a lot of speed. With single images, I might create a “foreground” and “background” layer and throw the appropriate layers in there. This helps in a lot of other ways, too – thoughtful layering makes it easy to lock parts of the drawing you’re not working on, for instance!
Rasterize finished layers. Target a heavy, finished layer by clicking on the circle to the right of its name.
Then apply effect>rasterize. Probably at at least 300dpi. Illustrator will turn this layer into a bitmap and use that for the preview renders instead of drawing it all from scratch. You probably want to lock the layer, too. When you are done with the whole drawing and want to do your final render, or need to edit something on this layer that turned out to not be finished after all, unlock it, target it, and use the Appearance palette to turn off the Rasterize effect. Warning: Do not do object>rasterize. If you do this then you will lose the original vector art and will have to copy it from an old backup of your file. You do have some kind of regular versioned backup system in place, right?
I call doing this “freezing” the layer, after a feature in an old vector program that did a lot of natural media emulation back around 2000 when even the fastest computer you could buy was probably less powerful than your phone is now. In Creature House’s “Expression”, you could lock a layer, and then if you clicked on the lock again it would change to a little snowflake to indicate that it was “frozen” – which meant that it was both locked, and that it was being rendered from a cached bitmap instead of the vector art. I think Expression did some additional work to make sure that any transparency in the frozen layer was handled properly with regards to stuff on layers below; Illustrator won’t do this, so a few things may look a bit odd.
This can be applied to layers that contain other layers, too, so you could rasterize entire finished panels, or otherwise stick chunks of finished art into one layer – say, if you’d decided the foreground was all done for now, just make a new layer called ‘fg’ and drag a bunch of layers in there. And if you are one of those masochists who works on a single layer and groups everything you can apply this to groups too.
A further refinement of this technique is to create a Graphic Style that’s just the rasterize effect, and apply this to layers you want to do this to. When you want to turn it off for everything, just select the Graphic Style, then visit the Appearance palette and turn off the rasterize effect, and do “redefine graphic style” in the Appearance palette’s menu. Then wait a few seconds as everything gets redrawn from the bitmaps.
Save without PDF compatibility. When you first save a file, you get some options. This is one of them and it’s always on. And I always turn it off. This is mostly just gonna effect your save times and your disc usage, but it can help a lot. Because when Illustrator saves a file with that on, it actually saves a PDF with all your effects expanded, and a copy of the AI file crammed inside it. Got blends, fill patterns, art brushes? Expanded into many more points and paths. Got bitmap effects? Turned into a bitmap. Which is saved in a manner that is in the running for inclusion in a list of Top Ten Inefficient Image Formats. If you are doing simple work then you’ll get files that are twice the size of an un-pdf-compatible AI file; complex work can easily end up 10x the size or more. And that will be reflected in how long it takes for Illustrator to save the file. It won’t affect the preview update speed like the rest of the suggestions in this post will, but it’ll still slow you down if you’re saving regularly. (Which you are, right? You’re not sitting there with a file saved for three days straight, hoping that the crash recovery will save your bacon if it crashes, right? Because it won’t always do that, even if you’ve gone to the prefs and turned off “turn off data recovery for complex documents”. Build that habit of hitting command-S when you take a break, when you finish a significant chunk of the document, or when Autosaviour pops up and reminds you that it’s been a while since your last save.)
As a bonus, sometimes files saved with PDF compatibility on will become corrupted in such a way that the AI file embedded inside is illegible. Illustrator will then load in the PDF. With everything expanded and unedited. And you will swear, and scream, and be very sad. Or at the very best you will sigh and go digging in your backups, and profusely thank Past You for making sure those were happening regularly without any effort on your part. Keep PDF compatibility off and this will never happen. You’ll need to generate a PDF when it’s time to interoperate with other programs; take care to keep those separate from the original AI files. Like in their own folder or something. And close the file immediately after saving the PDF version so you don’t accidentally decide to make a few changes that go into that instead of the AI version.
New splash screen. Don’t love it. Don’t hate it. Will probably replace it soon. I’d been seeing the splash screen credit for “Jade Purple Brown” next to the betas and thought that was just a placeholder fake name but evidently that is the name/alias of an actual person who has their own 
