I'll add my anecdotal data from a short investigation I did today.
Using Chrome / Firefox, the web developer consoles say that BO is "ready" after 1.5s. However when I sit watching blueocean load - it's actually around 6s to a usable screen with my jobs loaded. It takes around 800ms to get the main page, and then 900ms to download the jobs. And if I sit watching the consoles, then there is an unexplained 3-4s delay after the jobs are downloaded where BO is "doing nothing", and then everything renders.
In the screenshot above, the "idle" period is from 28000ms to 32000ms
I also found (in a separate run - don't correlate to above) that scripting is showing up as a major consumer of time
Obviously DOM manipulation has to be done - but I don't think I was expecting it to be such a heavy cost.
(I have a modern MBP with 4 processors and 16GB of RAM)
Enjoy this anecdote!