Can I perform multiple synchronous form submissions while the first of them is still being processed on the back-end?
It depends on the browser.
In latest Firefox (101) - it's not possible - only the first POST request is dispatched & processed.
In latest Chrome (102) / Edge (102) - it's possible - you can generate multiple requests by clicking submit / enter and each of it is processed.
All test performed on Windows 11.
To start testing: npm i
(installing dependencies) & npm run start
(running a server on localhost:3000)