Large Landscapes cause long PIE start time delays

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)

1. Download the attached Heightmap
2. Open the UE4 Editor and create a Blank template project
3. Delete the Floor in the Level and click on the Landscape tool
4. Click on "Import from File" and select the downloaded Heightmap then Import
5. Play in editor and then exit PIE
6. Check the Output Log for "PIE: Play in editor start time" and observe time is reasonable (around 1 sec)
7. Save the level
8. Close and then reopen the project and level
9. Play in editor and then exit PIE
10. Check "PIE: Play in editor start time" in Output log

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)