TL;DR: Downtime 2am tonight NZDT of around 2 hours.
Tonight we will have scheduled downtime while the host updates the hard drives on the server.
If you like technical details, the new hard drive array has an NVMe cache + full ZFS, compared to the current one which is an NFS share. This should cut out the networking layer and make hard drive access faster.
In terms of real world speed of the website itself, we may not notice much difference. Lemmy is heavy on the CPU particularly due to database access, but the database is largely held in RAM so we may not see a lot of difference in site speed.
It's likely to take around 2 hours, based on the host's migration of other services they host.
The host is https://fediservices.nz, which runs mastodon.nz and some other federated services. They don't run Lemmy.nz, but they donate hosting and so Lemmy runs on the same server as their services (so if you want to donate, that's who you donate to).
If you have any questions, ask away!