We manage two schools on using Mac Server and the network profiles (like roaming profiles) bring everything to it's knees.
In one school - because Apple laptops are rather expensive, each student doesn't have their own laptop. Every time a class fires up and logs in - the 30 laptops saturate the GB network for about 5 minutes. Until the darn things are done churning, the students can't do any work.
And yes, we've done all the smart things - pruned home folders, SSDs, awesome WiFi. It used to take 20 minutes before.
I send construction workers out in the field with MacBooks because they're so rugged and the RDP client it awesome, and we really enjoyed Apples line of plastic MacBooks. At the time, they were a great value for schools. I love apple stuff generally - I'm typing this on a Macbook right now.
But I'm absolutely convinced that Apple will lose the lower education space entirely in the next few years if they don't throw some effort into their server line (the software) and come up with a less expensive laptop.
2nd Rant:
Each new version of Mac Server makes fixing the Open Directory parts harder - in older versions you could search google to find the arcane chant to punch into the terminal to do magic. You could fix the underlying database, prune a user that isn't working well, and fix all sorts of corner cases.
Apple keeps removing these tools year after year.
Apple generally says that fixing the OpenDirectoy should amount to restoring from a backup - that would be fine if you found out about a problem right away, but if you've gone a month down the road it's annoying to potentially lose a months worth of work to fix a small problem. I haven't been bitten yet, but others have.
Whats the environment like for those network users? Are you only using 1 server? There's ways to load balance that. You're probably hitting throughput issues for sure.
On the second point: do you know whats missing from before? I'm pretty good with macOS server related terminal-foo. Might be able to help.