Sketch can easily deal with reasonably complex drawings, but if you end up with a big document you may want to know a few things about what is impacting Sketch’s performance.
Blurring layers is expensive. Sketch has to render the layer first into an offscreen bitmap (which is expensive), and then applying a blur on that (which is really expensive). The larger the blur radius the more expensive it gets.
For a 1px blur radius, Sketch needs to examine each pixel around each pixel; that makes for nine pixels to be examined per pixel to calculate the new average value. Increase the blur radius to two pixels and so on and it increases exponentially.
Note that Background blur is even more expensive than normal blurring is so keep that into account. If all you’re trying to do is blur an image, blur that, and don’t use background blur.
The same goes for shadows. Rendering (big) shadows on (large) layers is expensive, and more shadows mean more more delays. Inner shadows with spreads are especially expensive.
Sketch can easily handle a dozen Artboards on a page, but if that is combined with big shadows and blurs or even more artboards, things will slow down. One easy fix is to put some Artboards on another page.
Text to Outlines
Boolean operations are incredibly complex mathematical calculations to perform if you want to get it right. If you have a shadow with a few dozen subpaths, each with boolean operations, you’ll run into trouble.
This is why you should take care with vectorizing text. You can apply gradients directly on text without the need to vectorize them first, so keep that in mind. Other than that, if you can, put each character into its own text layer before vectorizing.