https://github.com/canonical-web-and-design/maas-ui/pull/1750 was implemented to address some of the issues you’ve found. Previously the UI was loading machines in batches of 25, it now (2.9.0b7) pulls in 25 for the first call, then 100 at a time in subsequent batches.
You can see the results of the investigation in MAAS Show and Tell: Improving UI performance for large MAAS installs