Hmmm...I think it is primarily a factor of the map size--how many worlds, pirates, militia, construction, combat, etc.
As you play, more and more current actions have to be written and stored into memory...eventually it gets full and your cpu puts memory loading on hold while it tries to catch up. Think of your memory like a chain of large dump trucks and your cpu as the excavator that unloads them. When you are going slow/freezing, there are so many trucks your cpu gets mental trying to unload them in sync and in real time.
Reducing the settings will help but if you have huge fleets and lots of pirate bases you might just be facing the limits of your hardware.
The other thing that can cause this behavior is high cpu temperature spikes due to the intense workout it gets in the game and insuffecient cooling. If you have an undersize fan this might be an issue (most likely this isn't your problem but it can be a possibility).
So far, the posts I have read states SINs doesn't support multi-core processors--it uses only one and XP only accomodates 3gig of your ram and some of that is set aside/reserved for the OS and all.
Last consideration is background programs--automatic updates, virus updates, downloads, videos and the like might be kicking in at bad times. I can't recommend going to hardware settings to optimize performance as I don't know what works best for SINs--maybe someone who knows the game software and comp hardware could comment on that.
My only suggestion is save the game and restart it--at least it can finish loading some stuff before it starts to crunch numbers again.