mmm well this is a complicated problem and I will try to give a simple and complicated answer:

there is a vanilla bug with saves. where performance is reduced by a small percent - usually tied to how large the save file is.

This problem can become worse when using mods which add data which is captured in the save file

UBE contains 10x the amount of vanilla data. so what was maybe a 2-3% performance decrease in vanilla , shows to be 20-30% decrease in performance after loading a UBE save.

Being that save game performance appears to be tied to the save file size, the following factors contribute to this:

Date and location starting a game in UBE (later years 1944 on) contain a significantly large amount of historical data - makes sense USA and more nations begin participating.

Other mods which also add Data to the save

Time spent in the campaign (the longer you play the more data gets collected ) < some of this is a vanilla bug to which data which is no longer relevant is still present in the save even though it doesn't apply to the current game state

Locals Data (translations and text) which is the biggest data segment of UBE and the vanilla game for some reason gets stored into the saves. It is a lot of unnecessary text saved in the file

Solutions:

Devs optimize the saving logic/process and remove unneeded data from save files (like Locals)

UBE optimizes - removes data which doesn't contribute enough to the overall gameplay to be maintained

UBE Lite which contains about 50% less data than UBE but still about 5x of Vanilla will help improve this.

lastly- Brute Compute force. UBE and even UBE Lite require a moderately powerful system (Memory and CPU , NOT GPU) in order to maintain optimal performance. UBE Lite is recommended for players which are running on laptops or have previously experienced performance issues with the game or UBE.



Simple answer.... Mod data gets put to saves> Saves grow over time > this hurts performance. It is a vanilla bug, exposed by a mod (UBE)

ok thanks, will then have to wait for a fix by the developers till I can use UBE again

Experiencing a similar issue, a pretty new game 3 patrols so far now on attack to scappa flow. Got attacked by 2 planes just before the shallow waters, after that on cruise again the game started lagging to 12 FPS in real time and 0-2 Fps in 12x.

What you describe should be a slow creep in performance slow down, or can it also be sudden like in my game?