**System Information**
Asus Sabertooth 990 FX | AMD FX-8350 @ 4.6 GHz | 32 GB RAM | GIGABYTE GeForce GTX 680 OC 4GB
Win 7 Ultimate x64 SP1
**Blender Version**
Broken: 2.73a final (Hash: bbf09d9)
Worked: unknown
**Short description of error**
As soon as the UV/Image Editor's left-hand "T-key" panel with histogram, waveform etc. is open and visible while rendering, the render performance drops significantly. Closing the panel immediately speeds up the render.
This is of course an easily avoidable issue, but if the user doesn't know about the interdependency of panel and render performance, he/she might easily dismiss Cycles as being unusably slow.
**Exact steps for others to reproduce the error**
Example .blend file here: https://dl.dropboxusercontent.com/u/27650383/rendertest_4.blend
Open the .blend file and render to UV/Image Editor. Open/close the left-hand panel by hitting "T" and observe the massive difference in render speed.
Results of my testing:
A. Rendertime T-panel closed
CPU: 00:32.02
GPU: 00:16.20
B. Rendertime T-panel open
CPU: cancelled after 5 minutes / 1697 of 15625 tiles rendered / estimated time left: 41 minutes
GPU: cancelled after 5 minutes / 21 of 256 tiles rendered / estimated time left: 57 minutes