I would like an answer from someone on the QBO side, please.

All right, every time in the past couple of years when I contacted support regarding how slow QBO was, I was always advised to use Incognito mode (Chrome). Now, I just happened to see a post where QBO advises that, to speed up performance, we should not use incognito mode.

Here's the post

Why is my QuickBooks Online slow?

which states in pertinent part

Many browsers today allow easy use of incognito mode. This mode can slow down performance for QuickBooks Online as each new session/visit to the site causes all site elements to be fetched again. Subsequent page loads in the same session should be faster but the initial page loads will be slower.