Tips to Avoid Performance Issues (Slow-downs, Freezing, Crashing, etc.)
For the most part, Inkscape's performance depends on how much RAM your computer has. While for some reason, the Inkscape developers have never released minimum system requirements for using Inkscape, I can say from experience that 2 gb of RAM is about the least you can have, and still be able to use Inkscape for simple projects.
Generally, I would say that with 2 gb RAM, you might see many of these performance issues in an SVG file as small as less than 1 mb. With 4 gb RAM, I'm usually just starting to see these hints of performance issues when a file reaches about 1 mb. If you're using filters in your SVG file, and a few other odd (and lesser used) features, Inkscape's performance relies on the computer's processors to display them.
But no matter how much RAM you may have, or how spectacular of a computer system you may have, it's always possible to find or create SVG files large enough to challenge your resources.
While you can still work in SVG files much larger than 1 mb, with only 2 to 4 gb RAM, you may need to be very careful about managing the file and what you ask Inkscape to do, to avoid a crash. I should go ahead and say this early on. Don't forget to save, save, SAVE your work. The larger the SVG file, the more often you should save 
Starting with Inkscape version 0.47, Inkscape has an autosave feature. In version 0.92, it can be found in Inkscape Preferences > Input/Output > Autosave.
These things can increase the file size, and potentially create performance issues:
- more than 2 or 3 embedded images
- certain filters or combinations of filters or the number of filters -- note that Blur is a very simple filter, but using a lot of it can still cause problems
- certain Extensions or combinations of extensions or number of uses of extensions
- large quantity of nodes
- a lot of gradients
- extreme zooming, and any zooming in combination with the other things in this list
- a large number of unused defs (excess and unneeded data in the file)
- using the Objects dialog (Objects menu > Objects)
- various combinations of the above
And here are some ways to either avoid these issues, or to continue working in a file, when you already have these issues.
- The first thing you can do when you start to notice any of these issues, is to click File menu > Clean Up Document. (Before version 0.91 it was File menu > Vacuum Defs.) This will delete any data in the file that is not in use. Especially if you're doing a lot of trial and error as part of your workflow, this data can accumulate, and slow down a file, even if there is very little content on the canvas. If this doesn't help, or help enough, move on to the next things.
- Divide your file into layers, and maybe even sublayers. Then you can hide all the layers that you don't need at the moment. Since Inkscape doesn't display hidden layers, it frees up resources for whatever you're working on.
- If hiding layers won't work, because you still need to see them, as you continue to work, try this.
-- Temporarily hide the layers you're working on, and display all the others
-- Select all
-- Edit menu > Make a bitmap copy
-- Move the new bitmap to a visible layer and hide the ones it's replacing
-- Unhide the working layers
-- When you're finished, don't forget to delete this bitmap copy - Use View menu > Display Mode > Outline. This prevents Inkscape from displaying gradients, filters, embedded raster images, etc.
- Use View menu > Display Mode > No Filters. This prevents Inkscape from displaying filters only.
- If you're using a computer with multiple cores/threads/processors (usually laptops) and have trouble with displaying filters, you can adjust some settings to better utilize the cores.
-- In version 0.91 and later, Inkscape Preferences > Rendering > Number of Threads. Before 0.91, Inkscape Preferences > Filters > Number of Threads.
-- On those same pages, you can change "Gaussian blur quality for display" and "Filter effects quality for display" - If you have a huge number of nodes, you might not need them, depending on the circumstances. If not, you could try Path menu > Simplify, which potentially could drastically reduce the number of nodes. In certain situations though, it can distort the path, so just be ready to Undo, if the result is not acceptable. Or it's possible to change the Simplification threshold, and have fewer nodes deleted at a time. (Inkscape Preferences > Behavior > Simplification Threshold) Note that selecting all the paths in the image, and applying Simplify will likely produce unacceptable results. It's better to apply it to one or a few troublesome paths at a time. Also, note that, starting with version 0.92, for the Pencil tool (which can create excessive number of nodes) there is a way to automatically simplify paths as you draw them, using the LPE Based Interactive Simplify option (look on the control bar).
- If you're using an embedded image as a reference, either delete it from the file as soon as you no longer need it, or move it to another layer where you can hide it. Deleting would reduce the files size much more.
- Avoid extreme zooming when possible, and zoom only as much as you need. If I have to zoom, and I know it will take a long time, I've found that I can scroll over to an empty area of canvas, zoom to whatever level I need, then scroll back into the area where I need to work. It's much faster than waiting, waiting, waiting, for Inkscape to reach a high zoom in a large file.
- Don't keep the Object dialog (Objects menu > Objects) open all the time. Open it when you need it, and otherwise keep it closed. (Developers are aware of this and planning a fix, but as of version 0.92.3, it's still a problem.)
If you have any comments, please feel free to post them below. Or if you have any questions, please post a message in the forum 
Brynn
8-17-15
(updated Feb 2019)
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