I haven't improved the speed, I haven't done anything.
I can just confirm what Fistalis explained, ie in my 2 previous games that became very slow to the point I had to give up, I have been able to see the situation Fistalis describes where you have a lot of units on tiny (a couple of hexes) islands. These units are moving forever because they try to destack (they are always more than 7 per hex), and according to Fistalis this is what can slow down the game pretty much. What happened specifically in my Venezuela game is the following: at some point I was at war with the US. The US have dozens of allies. All US allies deployed hundred of units as close as they could to Venezuela (but not IN Venezuela since they are not allowed to attack me directly, they have to attack from US territory, this is what is called proxy wars), ie on the islands that you can see north of the Venezuelian coast. I had 3 islands packed with units like Fistalis described.
Also, Fistalis explained how to load a savegame with another country than the one you were initially playing. So that gives you a mean to try to "solve" manually these kind of situations and see if there is an improvement. I haven't really tried that myself. I am afraid that even if you load as another country and either scrap all units packed on these islands or give them an order to move away, the IA might send units again as soon as you resume your game with your original country.
In my 3rd game, that I am still playing, I don't know exactly why, but the speed remained quite OK for a long time. I am now in 1968, I have conquered all of south Ameraica + the US, and the speed is around one minute per day (this is long when you are waiting for your nuclear plants to be built, but still acceptable, especially when you are at war, busy to manage your units, fleets, attacks etc.). Why was it OK this time and not the 2 before, I am not sure, even Fistalis said that he noticed different IA behaviours from one game to another. But I can say that I have not let the "island situation" happen again in my current game: by positionning fleets and submarines around the islands beforehand (because I knew what was going to happen) I just killed the dozens (hundreds?) of US allies merchant marine boats before they could land, then I just declared war to each and every island around Venezuela and annexed them
All in all, there are certainly several factors involved that impact game speed, and I truly believes that what Fistalis describes is one of them. Knowing that, we just have to wait that this "overpacked islands destacking issue" is solved in a future patch, or try to "influence" it like I did, or try to "manually solve" the issue as described by Fistalis.