My faction is TEC Rebel. Map: large random vast. All victory options are off. Unlocked teams, large fleets, active pirates, no militias, no factions, no super weapons, normal start. Faster for everything but culture, culture is set to slow, no random events. 10 player. 9 AI. 4 normal, 5 hard, 1 human (me). All factions variants loyalist and rebels of each 3 factions. The map is 9 hours 40 minutes of gameplay.
I've noticed a current problem with this large single player game. After a long period of time has passed. I'm not 100% sure as to what is causing this problem but I have a guess! The problem is related to diplomatic research of "Truce among rogues" and "Pirate mercenaries" with regard to their Advent and Vasari versions as well. After researching these techs and with the game progressing, pirates have come to my aid. Some of these pirate groups leave when they are done helping. However, some have lingered indefinitely. The majority of the pirates on screen are pirates that I believe other players (AI in this case) have given a mission for the pirates to attack a planet. It could also be that since the pirates have not been defeated, that they are still trying to war with me. As I have a standing peace treaty with the pirates (Truce among rogues). They can't attack my planets, nor can i remove them from mine. I am also unable to destroy their main base as they are "not seen as my enemy". They arrive at a planets, and continue to remain in them indefinitely. I believe this buildup of large quantities of ships is the source of what has caused a decrease in performance. It may not be the only factor, but I do think its the main cause, as the more pirates that arrive. The more lag within the game I accumulate. More pirates just keep coming to planets that do not have pirate fleets in them yet.
I had this game on CD since 2008 or 2009. I repurchased it when I found out it came to steam. Ive noticed that some patches were made to improve the game over the years and wanted to see if this is something that the folks at Stardock would be interested in fixing. If there is no fix then this can just be a warning for late single player games. My computer stats are below picture

OS Name Microsoft Windows 10 Home
Version 10.0.19041 Build 19041
Other OS Description Not Available
OS Manufacturer Microsoft Corporation
System Name DESKTOP-ORNSU0K
System Manufacturer System manufacturer
System Model System Product Name
System Type x64-based PC
System SKU Unsupported
Processor Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz, 4001 Mhz, 4 Core(s), 8 Logical Processor(s)
BIOS Version/Date American Megatrends Inc. 1202, 11/10/15
SMBIOS Version 3.0
BIOS Mode UEFI
BaseBoard Manufacturer ASUSTeK COMPUTER INC.
BaseBoard Product MAXIMUS VIII HERO
BaseBoard Version Rev 1.xx
Platform Role Desktop
Secure Boot State On
PCR7 Configuration Binding Not Possible
Windows Directory C:\WINDOWS
System Directory C:\WINDOWS\system32
Boot Device \Device\HarddiskVolume4
Locale United States
Hardware Abstraction Layer Version = "10.0.19041.844"
User Name DESKTOP-ORNSU0K\--------------
Time Zone Mountain Standard Time
Installed Physical Memory (RAM) 64.0 GB
Total Physical Memory 62.1 GB
Available Physical Memory 54.2 GB
Total Virtual Memory 71.1 GB
Available Virtual Memory 60.6 GB
Page File Space 9.00 GB
Page File C:\pagefile.sys
Kernel DMA Protection Off
Virtualization-based security Not enabled
Device Encryption Support Reasons for failed automatic device encryption: TPM is not usable, PCR7 binding is not supported, Hardware Security Test Interface failed and device is not Modern Standby, Un-allowed DMA capable bus/device(s) detected, TPM is not usable
Hyper-V - VM Monitor Mode Extensions Yes
Hyper-V - Second Level Address Translation Extensions Yes
Hyper-V - Virtualization Enabled in Firmware Yes
Hyper-V - Data Execution Protection Yes