I remembered there was a case in March 2017 where somebody else's SR300 was rendering at only 17 fps. In their case, they were running programs at a high resolution of 1920x1080.
RealSense stream programming expert jb455 wondered if the rendering speed might be being capped. Sadly, there was not a clear solution arrived at in that case.
What resolution are you typically running your programs and samples at, please?
I would also recommend double-checking whether your PC has the Windows 10 Creators Update installed, which the SR300 camera has been having problems with. If you do have the Creators update, a workaround for the issue has been published on this forum
Thanks for the information MartyG. The problem actually mysteriously disappeared this morning. But for anyone else reading this in the future, this was happening at all resolutions I tried, including 640x480. I'm not sure if I have Windows 10 Creators Update.
If the problem re-occurs, please do come back here again to report on it.
If you type 'system' into the text box at the bottom of your Windows screen, beside the button where you shut down, you can find the System Information app. This will tell you the current version number of your Windows. My own PC's build number below (15063) shows that I have the Creators Update.