This... is just not true. I use journald extensively on a 600Mhz Cortex-A9 daily and it's not slow. I don't know what kind of log load you have, but even when I have processes crashlooping and am sending logs over SSH over a VPN it's fine.
1475/1475 systemd:libsystemd / test-journal-verify OK 28.44s
but on x86 it runs in under 1s. Not sure the deal there.