Ahh that spinning wheel! The more paths you'll have, the more calculation Illustrator has to do to render them. And the more layers or anything like history and such will also count as extra work for Illustrator. Your issue is not related to the new Illustrator but to your own computer, how your files are built and your workflow.
If you have 500-1000 shapes, yes this can definitely be a factor that affects the performance of your software.
It doesn't help if each layer gets duplicated; even the hidden layers still count as data.
As you mentioned, you can use the "outline preview" to help yourself a bit but you might also need to join some shapes together with the pathfinder tool, "simplify" your paths and clean up your artwork. Keep what you need only.
One thing you can do before merging your shapes/paths together and simplifying them is to keep your original file with all the shapes intact in a separate file. Then use a new file with the simplified artwork. This way, if you ever need to go back, you'll have a backup file. This is a better way than using hidden layers. The hidden layers themselves are not a bad idea but in your situation, they're not helpful.
Already by doing these things, you should feel a great improvement when using Illustrator. Your other options is probably to get more RAM memory or change computer if you truly want the designer to keep the workflow he/she is using at the moment, OR use something as Indesign to "assemble" some parts of your artwork if that's possible.
Working with physically large (150''+) canvas sizes
Compress large PDF files in Illustrator