From a technical because-we-can standpoint this is really cool, but I am having a hard time imagining actual use cases that warrant running a JVM in the browser. For modern web applications it is too slow (judging from your paper), so perhaps abandoned legacy applications?
Have you encountered any practical use cases?
Yes; the paper mentions CodeMoo.com, which the University of Illinois created independently of us to teach basic programming skills to kids [0]. The file system component is readily used by the Internet Archive for their MS-DOS collection [1]. A number of other instructors approached me regarding using Doppio for in-browser IDEs, but Doppio is somewhat cumbersome to integrate into webpages, and never had the time to dramatically improve its documentation and ease-of-use. I believe that is a more fundamental barrier to using Doppio than its performance, especially since you can run Doppio in a WebWorker to avoid some of the responsiveness issues.
Also, note that Doppio is an interpreter, so there is significant interpreter overhead. Using a JIT compilation approach would improve performance, and a contributor is currently working on a basic implementation. I honestly believe that it could become significantly faster with additional work, but as a single person with other projects, I lack the resources to do this work myself.
[0] Doppio: Breaking the Browser Language Barrier paper from PLDI 2014: https://plasma-umass.github.io/doppio-demo/paper.pdf