20.17.0
Please try pre-release electron-updater 2.23.0.
Differential updates supported for NSIS (not only web installer). Please note — it is not delta, it is differential update. In short, it means that minimal update size it is block size of compression algo (LZMA2) but huge advantage is that updater is able to avoid full download to update to any version from any version. And this block size cannot be small because compression ratio will degrade.
So, if full installer size is 37 MB, and you simply change version, you can get results like (from electron-updater log):
File has 46 changed blocks
Full: 35,736.42 KB, To download: 917.08 KB (3%)
Please note — the same algo to compute blocks and download is used for all targets (AppImage, NSIS, NSIS Web Installer and soon DMG), but result varies because each target uses different compression algo and code signing algo (e.g. on Windows code signing can significantly change output file every time even if input file is the same).
Current implementation for NSIS has the one minor drawback — block map for current app version is downloaded every time and URL pattern should be the same (my-app-1.0.blockmap and my-app-1.1.blockmap, if you will change pattern for a new version to my-cool-app-1.1.blockmap, full download will be performed). WIll be addressed later.