Seems that we hit the concurrency limit in Chrome when downloading resources which causes loading to block on a cold load. We used GTMetrics's waterfall report to find this.
If we can combine all the CSS in JDL together then that removes 3-4 css files we have toninitially download and can let the browser download blueocean.js earlier.
- Concat all CSS in the JDL
- Update Blue Ocean to serve the concatinated CSS
This awesome tool provides some insight into what can be optimised (page speed + yslow + waterfall):
You can see from this, having fewer http requests wil allow the browser to do less "waterfall loading".
Once we have this, we can revaluate on hi and low latency connections (low latency things are much worse than that tool shows, about 10x slower for cold load).