Limit was added here:
https://github.com/discourse/discourse/pull/10125
We tried moving highlight to a dedicated worker (and consequently a background thread) but hit some issues around CORS.
If you want to lift the limit a good approach is taking this PR
https://github.com/discourse/discourse/commit/d09f283e911c0847ca23372aceefc344526690d2
and trying to fix the CORS issues it had and made it be reverted.