Matrix 2.0 will put an end to that trait plaguing the protocol's viability. With the new "sliding" sync, clients will be able to fetch stuff much more efficiently (and more akin to Discord's API, for example) than before. Syncing an freshly logged-in account takes seconds instead of minutes and syncing messages when returning to the app is instant. They presented this at FOSDEM this year: https://www.youtube.com/watch?v=eUPJ9zFV5IE