Operating system: Windows 10
Graphics card: GeForce GTX 1080
Broken: version: 2.83 (sub 11), branch: master, commit date: 2020-04-01 17:35, hash: a9963669f9d2, type: Release, uild date: 2020-04-02, 06:54:40
Many of my files created in 2.7x often spam the console with the following error when opened in 2.8x:
write_customdata error: layer '':15 - can't be written to file
Where '15' can be other numbers. Sometimes dozens of times whenever whatever is throwing this error updates, such as on a frame change, and blender is momentarily unresponsive.
I've found that through process of elimination by deleting data, and I've tracked down a specific example of a mesh data block that triggers the error so you can repro. Deleting the object does not stop the error, but deleting the mesh data immediately does.
I can't stress enough that this normally applies to dozens of objects in a complex file like a rigged character, so tracking down each one is a long process during which blender is extremely laggy.
Open attatched .blend file, open console, and change the current frame. Since I've only kept one piece of mesh data that causes it, you should see the error once per frame change.
Note: For some reason, switching into and out of edit mode also stops the error, but this isn't always practical in a complex file with hundreds or thousands of objects.