I've not raised a support call yet but will do so.
For info, I took some timings noted my experience with a couple of the example maps posted above:
https://experience.arcgis.com/experience/3c016ede34fc4936a91bd33c17f3b5c1
CHROME
Took about 5 mins 50 to load. CPU shoots to 100% when loading and stays there once map loaded, making rest of my system unusable. Even typing these observations into Notepad during the load is heavily lagging and the Windows clock I'm using to time it is stuttering and hesitating. Once loaded, map is unresponsive to user input and I would describe as being completely unusable.
EDGE
took about 3 mins 20 load, then experience same as above for CHROME.
https://experience.arcgis.com/experience/157c166b18e849a6aaa8606882d19453/page/Page/
CHROME
Took 6 mins 25 secs until finished loading. CPU at 100% during loading, making rest of my system unusable, but unlike the other map, once finished loading, the CPU settled back to normal and my system became usable again. However, map is again unresponsive to user input (eg. 30 seconds to refresh when panning and CPU shoots to 100%, making the rest of my system unusable until it finished refreshing the map). Again, I would describe the map as being unusable.
EDGE
Took about 3 mins 20 secs until it finished loading, then the experience same as abaove for CHROME.
I asked a colleague to test and they observed that on one device, the load times were ok but on another device (a more powerful desktop PC), it was taking minutes to load.
So performance seems to vary radically from device to device. Given nearly all the maps we (and presumably many other organisations) produce on ArcGIS Online are for public consumption, we need to have confidence they will perform at least adequately for all users and the range of devices they will be using. I currently do not have that confidence with the new map viewer and experience builder.
I will create a couple of examples of my own and log a call. Hopefully the cause of the problem can be found.