I am curious how you managed to transform it into a turn based game.
Well, I didn't. I just said "inspired by." Basically I love the sandbox nature of HoI3 and wanted to recreate that in the civ engine the best I could.
So, my design philosophy is a bit different than what you're doing with SoE. Instead of trying to recreate history for the player, essentially railroading events, I went in the opposite direction to allow as much alternate-history play as one likes.
Gametests of SOE long ago showed, that the parts of the US east coast were only interesting for human players playing the US. When the US was played by the AI, this part of the map only triggered big delays in the turn times and the AI mostly was not able to show a notable performance. In a first step I combined Britain and the US so the performance became a little bit better and traffic problems between both civs were solved. In a second step I cut out the US east coast completely, as this speeded up the game significantly.
One thing I really enjoy about paradox games like HoI3 is that they encompass the entire planet. In a WW2 game, what this means is that after all the fun is to be had in Europe, there's an entire world outside to expand & conquer into as an end-game treat. You're correct in that the larger the map & more units, the slower the turn times. Also there are limits to the number of civilization, buildings, resources, etc. I simply couldn't do a world scenario. So I picked a middle-ground between just Europe & the world and settled on including the United States: specifically up to the Great Plains as in the Cold War scenario. This way, as an Axis, Comintern or neutral player in Europe, you have the USA to look forward to defeat as an end-game foe.
In play-testing, turn times haven't seemed to be
too bad. Also, with Flintlock's patch the AI seems more adept at naval invasions. In the most recent version of Overlord, I've also increased the transport capacity of naval transports, as I read this is an easy way to make AI naval invasions much more dangerous (they don't seem to have a problem filling transports - it's multiple at a time that are a problem). I also gave the US player a base in the UK (and might expand this to more bases).
Another big problem was to operate such a scenario with normal techtrees. Each testplayer wanted to reach the ultimate units of a civ as soon as possible, making hundreds of nice units, at that time created by Wyrmshadow, in the game completely superfluos. I transformed the normal techtree Civ 3 players are used to, into a calendar. Now the units entered the game as it was in history with a small chance for the players to have some influence on the appearance of those units by speeding it up for three months.
So, I basically tried to port the HoI3 techtree into Civ3. It's the one bit of the mod that takes directly from the HoI3 Black Ice mod (besides the unit symbols).
It's the exact opposite of a calendar-like tech tree. It's the ultimate encapsulation of what it means to be a sandbox mod. A civ can choose to a large extent what it researches and when, as long as the prerequisites for the tech are met.
The progression is designed in such a way that late-game techs are locked behind long prereq chains. For example,
Heavy Bombers II is the tech that enables the building of Atom Bombs, but that tech requires most of the techs in the AIR section to be researched prior,
as well as techs in the STATE section under Atomic Research, which in turn requires late-game industrial techs.
So there's a rough historical tech progression that also allows great flexibility in what one wants to research.
The AI appears to deal with this somewhat adequately. There are a lot of techs, a lot of different paths to go down, and it seems to average out overall where they end up. Right now the Allies get "Great Library" wonders, but this appears like it's too fast of a tech progression. Still working on balancing this. Tech trading is off-limits to improve turn times.
Hopefully soon Flintlock will come out with an update to C3X that allows us to perfume techs!