Typekit provides a large library of fonts for easy use in web and offline sync. If using a font as a web font, the font size should – obviously – be as small as possible, and Typekit is providing the Dynamic Subsetting feature since 2015, which is a useful tool for keeping the size small.
Update from November 6, 2017: Heiko Mamerow ran a short test with Dynamic Subsetting and noticed, that his site made more requests and was a little bigger:
https://twitter.com/HeikoMamerow/status/925442356261933058
After that, I compared it on my site too and got a slightly heavier site for one of the two tested views as well. Because of that, I switched back to the CSS variant – Dynamic subsetting is not better in every case.
With Dynamic Subsetting, only the characters that are used on the current page are loaded from Typekit – that can reduce the file size of the fonts drastically. Typekit introduces this feature with the announcement of support for Chinese, Japanese, and Korean fonts, because these fonts are very large (multiple megabytes).
This option is not available by default for all kits. To activate it, you have to add a font which supports Chinese, Japanese, or Korean – for example, Source Han Sans Japanese. You will be asked if you want to convert the kit to a dynamic kit and get the new option after doing this, as shown in the screenshot.
Now you can remove the Source Han Sans font again and enable the option for the font(s) in the kit.