Since this is a new problem for you:


First do a spyware and virus scan.  You probably don't have any, but it's good to be certain.


Make sure you don't have any new programs, or updated programs, running in the background and hogging resources.  Check the windows task manager to see how much ram background services are using, and what your CPU performance is.


Defragment the disk, but first remove any programs you don't want/need.  The more full a hard drive is, the longer it takes the read/write heads to find anything.  As a rule of thumb, you don't want to be using more than half the total disk capacity.  Once you hit 50%, everything starts getting slower.  You should also run the Windows Disk Cleanup program to get rid of some useless junk files accumulated while web browsing or downloading music or graphics files.


If you have browser plugins like Realplayer and Quicktime installed, go into the program preferences and disable the options to load on startup.  They'll still work when you browse, they just won't take up memory and cpu space when you're not using them.  DivX is also a great one for using resources when it's not needed, so if you have it, disable load on boot for that too.


You might also want to get a Windows registry cleaner if you don't have one.  I use CCleaner (downloaded it from zdnet.com) for the registry and to get rid of even more unneeded temporary files that Windows disk cleaner misses.  As with any registry editing, just be sure to create backup before running it.

The game slows down when there are a lot of ships being built and a number of them are moving.  I notice this starting at around 5000 huge hull warships.  Are your ship counts in this range when you see the slowdown?

My basic specs are Vista Business 32 SP1, 2 GB RAM, E7200 CPU @2.53GHZ, 160GB HD and 512MB GTS250. 

My machine has no extra programs running, so that's not a factor.  I built the machine to play GalCiv2 and it does nothing else.   It has a fresh Vista install on a new hard drive from 2 days ago and nothing besides Avira AV, Malwarebytes AM and GalCiv2 on it.  Before that, it was XP Pro SP3, the AV/AM and GalCiv2.  Nothing else.  I even shut off the network adaptor and kill extra processes unless I'm doing an update, starting a new MV game or uploading a finished one. 

I set my graphics to the "poor" preset in game and zoom out to the icon view while playing to help get around the problem.  Also, before I hit end turn, I zoom to a place on the map that has as little going on as possible.  This is still in the icon view, but not zoomed all the way out.

Once the ships stop moving, it gets better.

Another thing that will help is to fleet up your ships as much as possible.  A fleet of 5 huge hulls is handled like a single ship, so that's like having 5x fewer ships wandering around.

I hope this helps.

glad I could help