If the hardware used (most notably the drives) support encryption natively then all encryption/decryption is handled on-the-fly by the drive electronics. This is very common in better SSDs (including most/all of the Samsung drives), search for AES on the spec sheets for the drive.
SAS drives that support encryption are available, you'll probably need to get the precise model number you want using a configurator or spec sheets on the manufacturer sites.
I have no idea if there are non-SSD SATA drives that support encryption natively. At that point you're probably looking more for Bitlocker and the like.
On modern-enough equipment and OSes there are things you can do to let the machines automatically retrieve a key from the network at boot time as long as they're on the internal LAN - while being encrypted and protected if the machine is taken outside. I have a place that I'd like to look at that for, but I think that the tablets they'd like to do it on are probably too old. It's called Network Unlock:
https://blogs.technet.microsoft.com/dubaisec/2016/04/14/bitlocker-network-unlock/ and I have no idea if there's a way to do something similar on servers/VMs.
I used to think about the possibilities for encrypted systems that would require a response from a network device (easily hidden) for unlocking, I'm honestly not sure if that predates Windows 8 or not. Certainly it doesn't predate when work would've needed to start on this.
edit Re: system load, at least with modern Intel Core and Xeon processors it shouldn't be an issue as they all have the AES instruction set built in so AES-based encryption should fly. Not sure about the cheaper Pentium/Atom/Celeron/whatever processors or AMD stuff, but on the Xeon side it's been there for probably 7-8 years?