Ok @eviltrout this is sorted now per:
https://github.com/discourse/discourse/commit/8745f78277f80250032352343bde59f7bba12bec
sam@ubuntu discourse % ruby --version
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux]
sam@ubuntu discourse % bin/unicorn
I, [2019-02-21T17:21:45.013068 #12680] INFO -- : Refreshing Gem list
Starting CSS change watcher
WARNING: Discourse development environment runs slower on Ruby 2.5.3 or below
We recommend you upgrade to Ruby 2.6.1 for the optimal development performance
I, [2019-02-21T17:21:46.552725 #12680] INFO -- : listening on addr=0.0.0.0:9292 fd=11
Starting up 1 supervised sidekiqs
Loading Sidekiq in process id 12802
I, [2019-02-21T17:21:50.802457 #12829] INFO -- : worker=0 ready
I, [2019-02-21T17:21:51.082014 #12680] INFO -- : master process ready
I, [2019-02-21T17:21:51.861961 #12906] INFO -- : worker=1 ready
I, [2019-02-21T17:21:52.744207 #13036] INFO -- : worker=2 ready
If you want your dev instance to be performant I recommend upgrading to Ruby 2.6.1.