There are three ways to do this, each is a different blend of compromises.
The first, would be the easiest, is to use shouldRasterize on the existing labels. This doesn't seem to exist for labels in Sprite Kit. DOH!
Use bitmapped textures as letters on objects, actually as sprites, the thing that Sprite Kit handles best. This will involve using a bitmap font generator, such as the excellent BMGlyph as pointed out by Whirlwind in the comments.
This will give not be easy because the coding part will be a little more labour intensive, but you should get the absolute best performance this way.
You can swap letters, too, still, but you will need to think of them as subsections of the texture rather than as letters. An array or dictionary with each letter's position in the texture assigned to something easy to remember will be both performant and easy to use. But labour intensive to setup. Much more so than SKLabelNode
Or, you could go wild, and create textures with code, by using an SKLabelNode on a virtual object and then "rendering" or "drawing" that to a texture, and then using that texture(s) for each letter onto the objects/sprites. Similar to how BMGlyph works, but MUCH more time consuming and much less flexible.
BMGlyph is the best combination of speed and ease of use, and it has some pretty fancy effects for creating nice looking text, too.
