ts_zip ( https://bellard.org/ts_server/ts_zip.html ) already implements lossless compression with language models and is fast enough to be usable for some applications.
FYI, the corresponding standalone Linux command line version is available at https://bellard.org/nncp/gpt2tc.html . It also does text completion and file compression.
To give some perspective, the setup of http://textsynth.org consists in a single C Linux executable of 250 KB on the server and in 150 lines of Javascript code on the client without any dependency on other libraries...