I suspect Windows 10 is the culprit because I've read that there were some changes specifically to DX9 support that can greatly reduce performance.  But, that degradation is due to a specific startup parameter that I am now logging and I will want you to load an ERAS storm save and show/send me the system.log files.

I have just updated Itch with some updates and this new logging.  Here is what I want you to try

1. Load the ERAS storm save with 64 bit version, let me know if this has degraded FPS and give me the system.log

2. Load the ERAS storm save with 32 bit DX9 version, let me know if this has degraded FPS and give me the system.log

3. Load the ERAS storm save with 32 bit DX8 version, let me know if this has degraded FPS and give me the system.log

I am also noting your updates particular to foliage and possibly the sailors.  If you want to try disabling the crew in Caribe Stories to verify, edit a script file in Program\SEA_AI\sea.c and comment out this line with a //:

//CreateEntity(&PeopleOnShip, "PEOPLE_ON_SHIP");    ReloadProgressUpdate();

That should disable all crew on ships.

I don't think it's the actual execution logic of foliage or crew that slows, but likely some particular render state/shader code that greatly hampers within Windows 10.  Even with the faster video cards, faster GPU, whatever they did to DX9 support within Win10 mucked something up but I somehow need to find out specifically what to see if we can circumvent it.  This stuff is old and it is technically not complicated enough to hamper and slow new GPUs, so it is quite strange.

Just an example of one thing I'm looking for:

https://walbourn.github.io/windows-10-anniversary-update-sdk/

"Direct3D 9 Users: Note that with the updated operating system, there will be some performance impact if you are using D3DCREATE_SOFTWARE_VERTEXPROCESSING or D3DCREATE_MIXED_VERTEXPROCESSING"

The game does query the capabilities and automatically detects and uses D3DCREATE_HARDWARE_VERTEXPROCESSING,if at all possible, but I'm curious whether the query comes up with the wrong answer in your case, so that is one of the things I will show in the log is what value the game uses at instantiation.

So far, with Win7 and Win8 and Win10 tests I have done, have not seen this problem, but I have also heard that some of the high-end video cards, with Win10, for some configurations of equipment/OS, all of a sudden go super-slow with this old DX9 stuff.  So not everybody, just some cases and there are various examples within online forums with this same type of problem for some old games.  I will try to figure it out, but I only have two Win10 machines in my household, but both have the stock on-board video cards and neither have this problem in the storms when I recently checked, so it may be difficult for me to figure out the cause.