So I was tying out unreal 5 preview 2 and thought I would have a go at converting my world from world composition to world partition . I looked at some videos and found there was an option to convert level in the tools menu. After a couple of tries which took a long time and got nowhere I set it to verbose logging, dont convert inplace and left it running. It spewed out some errors to start with ( as it was preview 2 all the plugins had been disabled so some blueprint object didnt work I think) . No biggy Im sure it can ignore those actors 
My world is 20 km by 20km is split into 4 landscapes at 2m tile resolution.
Although I have painted around the coast its still mostly one of 2 textures (well materials) for about 80 percent.
So although it took several hours to start to partition the world and I think it took a couple of hours to partition it (on the log) what really took the time was the rendering the landscape tiles.
Now I dont have the worlds fastest graphics card (gtx 970, but I pretty much do have the worlds fastest processor (5900x) and it took over 4 days to render my 400 tiles.
Whilst it was doing this it wasnt actually creating any files I think, or my HD space didnt change much. Though my pagefile (different ssd) in windows went upto 130 gb.
And the end result of all this a handy message “Conversion failed!”. So I dont even know where a message log is or why it failed. I wouldnt be at all suprised to find it failed on the errors generated in the first few seconds and yet it carried on trying the conversion for literaly over half a week ! Whatever it was doing rendering those squares was a monumetal waste of time and unbelievably inefficient. And probably was just rendering blank black squares and blank blue normal maps anyway. What it was doing that took so long looked like the sort of thing where it generates distant proxies for grid tiles for levels.
As Im doing this with the forward render what I can tell you is that that doesnt work normally anyway, and just produces black squares, so I highly suspect that this would just be rendering black squares too. All it wants to do is take a picture of it from a distance, that should take 0.01 seconds not 4 days.
So in summary :-
It doesnt work, it takes waay too long to find out it doesnt work. It spent 4 days and I think created literaly nothing !
There were no grid partitioning options visable when I ran the commandlet, so I dont know what size it was event trying to partition it to.
I dont even know where to find why it thinks it doesnt work.
And given that I was doing this with the forward renderer, all that time spent generating distant textures would probably just have created blank black squares anyway.
So it needs some option to say how long it is expected to take, some option to turn off or massively speed up the grid tile rendering, some indiciation of where a log file might be found.
As it takes sooo long to do playing around with options isnt really much of an option at 4 days between tries!
p.s. if you do it from the command line does the -AllowCommandletRendering create those tiles that create ages and if so does it partition the world if you dont include that option ?And why isnt that option in the tools menu version?