I have a theory about why Blue Ocean page loads can seem so slow. If you look at the last, 3rd last (the highlighted one), 4th last and 7th last items in: you can see that it takes 30s (yes, 30 seconds!). Every Blue Ocean page load does this, AFAICT. It seems to be some kind of GET that is meant to take 30s for some reason.
The problem with this kind of behaviour by the (Jenkins web) server is that modern browsers limit the number of sockets (6 for Chrome, IIRC) that are allowed to be open to any single server. When you have a page that ties up a socket for 30s, repeatedly in a "live refresh" (i.e. AJAX) type of web page it doesn't take too many tabs open to Blue Ocean pages to tie up all of the limited number of sockets the browser will open to a server with these 30s page loads. All other fetches to the Jenkins server then block waiting for one of these sockets to come out of their 30 GET.
This can be seen by going to a Classic View Jenkins page and watching the status bar on the lower right on Chrome saying that it's waiting for available sockets. I get that very frequently when I have a number of open Blue Ocean pages. And it goes away as I close down the number of open Blue Ocean pages.
Having a limit on the number of open Blue Ocean pages should NOT be considered the solution to this problem.