Have you dug into the settings? I hated the change too, but found that the aspects I most hated (mostly the hover buttons that made everything jump around when used with the preview pane) were configurable.
The aspect I most hate is the load time. Takes about 6 seconds to start displaying emails. Is there something to be done about that? (Note: I'm using Firefox on a Mac.) I've been mostly using the "basic HTML" version, which loads in less than a second.
I'm using Firefox on Windows 10. Loads in about 1.5s for me. Obviously it'll depend on PC and internet connection. I would try it with addons disabled though. (Restart with addons disabled / in safe mode in the help menu.) In the past I've found misbehaved addons (particularly the god-awful skype one that used to always force install itself) would bring gmail to a crawl.
