Synapse's readme does try to spell out that it requires at least 2GB of RAM and a recent CPU. It is absolutely resource heavy, but not showstoppingly so in general. The comparison with XMPP is dubious as the protocols are completely different: it's like comparing a local filesystem with a distributed database and complaining that the DB is slower.
That said, Dendrite should improve things a lot; we should have more stats in the near future but it seems to idle around 150MB of RAM and should run much better on ancient hardware. We are not bothering optimising synapse much further in favour of focusing on finishing Dendrite. Needless to say, we expect Dendrite to be finished well in time for the Librem 5 to go live, 18 months from now!
That said, Dendrite should improve things a lot; we should have more stats in the near future but it seems to idle around 150MB of RAM and should run much better on ancient hardware. We are not bothering optimising synapse much further in favour of focusing on finishing Dendrite. Needless to say, we expect Dendrite to be finished well in time for the Librem 5 to go live, 18 months from now!