What are your system specs? This sounds like a graphics slowdown, which you can verify by hitting F3 and then looking at the lines "Last Turn Avg Process/Render MS" and "Max Process/Render MS." If the process is really high, then something is up with the CPU usage, if the render is really high, then your GPU isn't holding up. I suspect the latter.
You should be seeing a Last Turn Part/Full MS of somewhere around 160/200, or 200/200 (this is the compound number of the process/render numbers above). If that is significantly higher than 200, then that's basically telling you your simulation is running at a reduced rate because your CPU or GPU isn't keeping up. That would probably be at 400 to 600 for the slowdown you are mentioning.
I do note that the planet you are indicating in your savegame (which runs fine on my machine), is a ringed planet, which has a lot of alpha blending. Some players with onboard graphics cards have reported slowdowns with such planets, although this is not something I have heard much, or at all aside from those with integrated/onboard graphics cards. If you want to avoid that, you can simply turn the planet graphics off under Settings, Graphics, Backgrounds, Disable Planet Display. If your GPU is struggling, that would be the solution for it.
Sorry to hear of your troubles, hopefully that helps! This slowdown would not have been new with 1.301, it would just be chance that you happen to be encountering it now -- the planet graphic in question has been around since 1.201.