These delays appear with large Landscapes (for example 8029 x 8029) . The delays only start occurring after the level is saved and reopened. After which PIE start times will be 5 to 15 times longer than after creating the landscape. If a landscape is in a sublevel, moving it to the persistant level will reduce PIE start times back to normal, until the file is saved. The delays will not appear when creating a blank landscape, only landscapes created from heightmaps cause delays to appear.
PIE load times began to spike with Heightmaps in 4.23.1 and behaved as expected last in 4.22.3 (CL-7053642)
This was reported and tested in 4.23.1 (CL-9631420). This was reproduced in 4.24.1 (CL-10757647) and Main 4.25 (CL-11152609)
Results: PIE start times are much longer after saving and reopening a level with a large landscape in it.
Expected: PIE start times would remain consistent, as older versions of the engine did (4.22.3)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-87701 in the post.