Sketch can easily deal complex drawings, but if you end up with a large document, you may want to know what may be impacting Sketch’s performance.
Adding many images to a Sketch document may slow down general navigation, but it can increase the filesize dramatically. Especially when larger images have been imported, and resized to smaller dimensions. A handy workaround to this, is to periodically Reduce Image Size on your document after you’ve added many images, which will help with improving its performance.
Applying many blurs to layers may cause Sketch to slow down. The reason for this is because Sketch needs to render far more pixels than a non-blurred layer. The larger the blur radius, the many more pixels Sketch needs to try and process.
For a 1 px blur, 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 or more and it’ll begin to increase exponentially.
Shadows, both inner and outer, are treated in much the same way as blurs. Sketch will try to improve the performance of large shadows when zooming in, by only showing them when necessary. If viewing a document when zoomed in past 400%, shadows with a large blur radius won’t appear in order to speed things up — but they will appear as expected when you export them.
Sketch can handle a good amount of Artboards in a document, but when you start to add many across the infinite Canvas, and if they nearly all contain large images, shadows, or blurs—then you may see a drop in performance. A simple remedy is to take advantage of the Pages feature by including some Artboards on different Pages.
Convert to Outlines
Perhaps the number one cause for slow performance is when vast quantities of text has been converted to outlines. This is a great and useful feature for vectorizing various words and characters, but it is not recommended to convert entire sentences and paragraphs to outlines.