That's strange. When I open a 1m+ LoC in Rider it takes around 2GB of RAM [1] but is still very workable. Ofcourse it is not extremely fast but I am always impressed this even works with all the code hints, indexed code and what not.

By the way, sometimes a corrupted cache can be a problem. You can fix this via File -> Invalidate Caches.

[1] AMD Ryzen 9 3900X, 32GB RAM, NVMe disk. Ubuntu.