For context, I've been using Aternos to run a 1.16.3 paper server with a few friends for around 3 months now.
Even if we did experience some kind of lag, we could always lead it back to something changing within the server, and therefore fix it.
Yesterday this quickly changed, as we went from a consistent 16 tps if everyone was connected and in the overworld to a wildly varying 2-12 tps if I'm the only person on, quickly dropping to an average of 5-8tps if anyone joins.
Nothing changed in the server's infrastructure, used command blocks or installed plugins, I've even removed some that were unecessary for the sake of boosting TPS, but so far nothing has changed.
Any aid in fixing this would be greatly appreciated.