Just to jump in the ring and bring my 2 cents worth of experience 
I currently use a Xojo web app as an API backend to handle around a million https rest connections per day with a fair amount of data. I understand it’s not the same as having a user facing web app, but my setup is fairly scaleable and may help seeing that some things we think cannot be done may be accomplished by having a good architecture where Xojo is a part of the solution and other pieces can be incorportated.
I have a 2 (for redundancy) haproxy load balancers facing the internet runing on Debian. They handle SSL termination and pass the connections to the backend servers.
I have 5 backend servers, all Mac Minis running MacOS. Each server has one instance of my webapp per physical core to take advantage of all the power in the machine. The Minis do a lot of local caching so that they each will hit the database once for a given information and they will hit the apache server once for any given media file.
I have 2 Postgres databases, one master and one hot standby.
I have an apache server to handle static media files (mps audio and mp4 video) and some static text configuration files.
My setup scales fairly well. I have done it and will do it again as our traffic increases. Simply add more Mac Minis as backends and add them to haproxy load balancers. The database can be replicated and the load distributed among instances. The apache server is fairly idle as most files are requested only once per backend instance and then cached.
In my opinion, no matter the technology/language/framework, the most important thing is to plan the setup to be scaleable from the start, and no single tool will be able to handle all the required parts.
Again, just my two cents.
Again it’s not the same as a full GUI Xojo web app.
But my setup works, and has been working for years, reliably. And I would dare say the project of the OP could be done with Xojo. It’s not a matter whether Xojo can do it or not, it’s a matter of planning the architecture. As it would be with any software or language.
… now I’m ready to be shot at !!! Remember, I’m just a guy trying to help !!