SSH and LuCI seem to stop working properly while CPU under high load

On 21.02.0. When performing a backup using dejadup to a Samba4 share, the LuCI interface starts throwing 502 bad gateway errors and SSH sessions get closed immediately after supplying the password. I'm on an ipq40xx target that has 512MB RAM with irqbalance enabled. Is there something in the config I should set, or maybe use a nice value for Samba so this doesn't happen?

I'd rather set a nice value on ssh.

That does htop say about the CPU load while copying?