The slowness issue in Joplin synchronization is a known problem that users discuss. You can see an example discussion on this topic here:
Someone named @JackGruber claimed to synchronize all their notes in 5 to 8 seconds using Webdav. However, he ignored my question about how to set up Webdav. I think either they don't want to explain it or have stopped participating in the forum; I don't know. However, I'm leaving a link below for you to explore this path: