ID 352161 - UI freezes when loading large textures in viewer shaders

Problem summary

When loading large textures in a viewerSurfaceShader for the OSG viewer or hydraSurfaceShader for the Hydra viewer (starting at about 4K resolution), the respective Viewer tab shows a noticeable lag and the entire UI can freeze for a few seconds.

Customer reported version
Katana 3.0v1

Customer reported platform
centos7

Steps to reproduce

1) Drop a PonyCreate node into the Katana node graph.

2) Create a viewerSurfaceShader or hydraSurfaceShader material and choose a shader type that supports textures.

3) Load a smaller image as a texture.

4) Add a GafferThree node to the node graph and create a light with a viewerLightShader or hydraLightShader.

5) Expand the light location in the scene graph and position the light so it points at the pony. You should now be able to see a preview of the material in the Viewer or Viewer (Hydra) tab.

6) Edit the material and load a texture with a large (16K) resolution. Now try to interact with the Viewer tab. Result: the Viewer tab will be frozen and the UI may become unresponsive for a few seconds.

Workaround
Unknown.

Reproduced by support
This bug has been reproduced in:
Katana 3.0v1 - Windows 10 - CentOS 7
Katana 2.6v4 - CentOS 7
Katana 2.5v1 - CentOS 7
Katana 2.1v1 - CentOS 7
Katana 2.0v1 - CentOS 7

Expected behaviour
Textures referenced in viewer shaders should be loaded without freezing the Viewer/Hydra Viewer tab.

Actual behaviour
The Viewer/Hydra Viewer tab freezes when loading large textures.