@Greg OLone Right. So the problem is that there’s distance between where you’re doing the drawing and the browser. Where the desktop is instantaneous, each round trip to the server could easily be 100ms. That’s just the fact of the internet.
It does mean the bulk of your user-interaction needs to be programmed in something other than Xojo, of course. If you already have such a tool (for example, using janvas ) you can use as a stepping stone then it would be easier (be aware of license issues).
This way instead of drawing in a Xojo Canvas you'd be really drawing in a local "canvas" and sending the resulting vectors to the Xojo App, which should be able to store them.
Likewise, if you send the vectors back to the web ("opening" the drawing) you should get to the same end state.
As Greg says, WebSDK could be used to build a custom addon for Xojo that does this translation. You can technically use any external tool that can run in a web browser, providing only the "glue" to interact with your back end.
Another option of a drawing program back-end that could be customized for the same needs (they all output SVG, which can be the "format file" exchanged with the back end)is DrawerJS: https://www.drawerjs.com