I've never had this problem.  I downloaded your save to see if I would have the problem and to see what the problem looks like if it did happen to me.

It did happen to me with your save.  It was horrible.

I was checking some things and noticed that the turns left for the ships were decreasing, so I let things continue.  After about 6 minutes 15 seconds, most of the Torian ships had reached zero turns left and had stopped moving.  There was a probe in the entrance to the subspace stream with turns left, but it wasn't moving.  I tried using some console commands to delete it but failed.  Also, the turn was still stuck without the Torian ships moving.

I loaded your save again and used console commands to delete all the Torian ships that were using the stream.  When I went to the next turn, the game continued without a problem for 6 or 8 turns before I stopped playing.  I restarted, loaded your save, deleted the Torian ships and saved the game as Test Updated.  I restarted, loaded the updated save and continued without a problem for several turns.

If you want to try to continue, that save is available here.  I have no idea how the game will change with the Torian ships deleted.  I also have now idea if the problem will happen again.

I hope the Devs can use your save to find the problem so it can be fixed.

I think the problem is caused by the fleet being on autopilot into a slipstream, the other end of which is outside the travel range.

I had been plagued by long waits between turns with no clue why until I noticed this happening to one of my own fleets.

I managed to click on one and redirect back to my home sector and that fixed it.

I've had this happen while playing against the Corporate Sector.  They just shuffle what must be 70 or 80 tiny hulled ships between subspace conduits and it creates horrible game delays.

Repros for me too. Just like you say the Ai is having trouble trying to enter a stream. Im going to get a developer to check it out.

Here is my save game doing the same thing if that helps 

https://drive.google.com/file/d/1j6TIV9u9gXoCIWXUacO8xn7BEveZF8W1/view?usp=sharing

I've just run across this bug too.  Very frustrating but I'm sure the wizards will fix it and then all will be well.  In the meantime, this does allow me to accomplish some things around the house.  Bathing, eating, sleeping, yard work, going to work, etc etc,etc.

I think I may have a clue as to cause:  In my own ships it only seems to be appearing in ships whose final destination is NOT in the sector they are entering.  My next-sector traffic appears ok, but my long-range travelers (freighters mainly) seem to get stuck.  I've been forced to decommission those ships in the interest of my sanity.

I've noticed this as well, it only happens when the route crosses three or more sectors, ie, passes through at least one sector, meaning goes through at least two slipstream lanes. The thing though it that it doesn't happen in ALL such cases and sometimes going back the other way along the same route doesn't produce it either and to top it off, there are cases where I might expect it to happen and it doesn't. It doesn't feel random to me though.

May latest solution is going to war with the identified race(s) causing the slowdown, so I can destroy the offending ships!  LOL

So, I had MAYBE this happen to me the other day. Fortunately, I have Autosaves every 2 turns, so it didn't affect me as much beyond the irritation of having to force quit the game. When I restarted it, I did so with the console enabled in case I had to try deletions or just to see what was happening since it was on Unknown Civilization 14 that the game hung up at. Now, I don't know if the console being enabled caused some difference in AI actions, or the reason was behind a Pirate/Peacekeeper/Monster cluster that moved differently this time around or what, but it didn't repeat.