Strange, my experience with Inbox is nowhere near that bad. It is noticeably slower than Gmail classic, but load time is ~2s and actions like search take a fraction of a second. This is all on a 2012 MBP.
Gmail is slow to load compared to others. Notably, FastMail. I abandoned Inbox a while ago because of load time. Coming close to abandoning Gmail too. Takes way to long to load. This happens for me on low end laptops AND high end desktops.