Host from germany here - I am having the same problem. On mobile phone, the inbox loads reasonably fast (not lightning fast, but its okay), but on desktop the inbox can take as long as 10 or 12 seconds to load. I get 5 to 15 messages per day, and my policy is to always answer my guests to be a good host. Oh how I wish I could just click and answer. 😉
I have stomached it for months now, but now I needed to vent my frustration somewhere. I have a high-end gaming PC and extremely good connection, and other sites load blindingly fast. But the inbox (and to a lesser extent, the calendar) are a pain to use.