Interesting as my single thread speeds have been consistently in the 30-45 mbps since at least the beginning of September. It could have been earlier as I don't have any saved TBB results for July & August. No variety with time of day though.
You haven't recently changed your hardware have you? The reason I ask is that the last two laptops I owned struggled with single threaded tests (wired and wireless). They spent most of the test ramping up. I was never able to find out why. Then I had to replace the laptop and the new one (an HP Pro Book) goes straight to full speed on both tests.
be some aspect of the test that causes it to give poor readings on some hardware. I vaguely recall it started after one of the Windows 10 updates for what that's worth.