TerraDave
5ever, or until 2024

So I think we had at least three threads on the strong opening for XGtE, but setting aside whatever "fastest" means, it never quite got to #1 overall on Amazon, like the PHB did.
The PHB is back on top, possibly as a new semester brings new campaigns. But with XGtE D&D now has 7 perennial products pretty much always posting decent sales. Possibly for years into the future.
PHB #65
XGtE #112
MM #245
DMG #262
VGtM #706
I expect XGtE to float down to somewhere between 200 and 300 and stay there for a long time.