Yes if you’re computer barely meets minimums or doesn’t meet them, then that would explain your slowdowns. UE4 has a new LOD system as feature page notes, so I’m a tad surprised you are seeing such issues. What size terrain did you choose to create, the largest one ?
I’m pretty sure the default terrain system does not use paging, meaning terrain segmengts aren’t swaping in and out as player moves, thereby alieving you’re rig from performance problems, so to remedy that you should be using the new landscape mode tool which does enable ‘tiled’ terrains which would make a big difference most likely , though Ive not tested it yet as I couldn’t use world machine to test tiled terrain import and use as I can’t afford the pro VERSION which is very closely for me.
Terresculptor alpha version willl not export tiiled terrains, and Im not sure what the authors plans are, so all that leaves is L3DT which I think ? exports tiled terrain and is free to use for personal or commercial purposes, yet is limited ( if you see it as a limitation) to I believe, 2048x2048 sized terrains.
I am not positive l3dt supports tied terrains, but it seems to me it does, I"ll check to be sure and update this thread. At the this time therefore, L3DT is easily ( If im right about tiled support and even if not) the best option for budget conscious individuals here, including myself 
There are other tools that handle tiled worlds I think, but I don’t have info about them offhand, I’ll try to find it.
Good luck all, this is certainly a challenging aspect to our world creation, but very doable once you get the hang of it.
One last thing that would be valuable for those who haven’t made such worlds, are a set of pictures for creating the LONger VERSION of making a world material in UE4 ( for splatmats , in use for imported terrain) vs the shorter method which is MUCH easier, but which reportedly anyway, offers less visual fidelity. As time goes if no one has, I’ll try to make this happen.
Cheers
nl
At