I take it as a given that BaseX, properly configured and using queries that are not poorly implemented should perform acceptably as a concurrent web server with not too much load (10s or 100s of users, not 1000s of users).
Absolutely. Our usual benchmark for commercial applications is to ensure that 90% (or more) of the queries take less than a second, and to only allow non-blocking queries to take more than 5 seconds.