System Information
Windows 10, quadro M6000 + Gtx 780

Blender Version
Broken: (2.76b f337tea)

Short description of error

The previews_dynamic_enum slowdown blender when we use network path for the library with thumbnails.

There is a little video to show you the difference between, local path and network path.

http://pitiwazou.com/screenshots/bug_report.mp4

Exact steps for others to reproduce the error

In our addon we add a path in absolute to our library on our PC.
That show the preview of the thumbnail in the T panel.

When we use local path, blender is fast
When we use network path (on my NAS in Gigabyte Ethernet with agregation), blender is really slow, unresponsive.
When we hide the preview, blender is fast, so we think it's a preview issue.

I tested with asset flinger (opengl) and that works fine even with the preview, but not with the previews_dynamic_enum from blender.