Can I give you a tip? The same has occurred to me a number of times. Usually by the time I get 5 or so years into the game. What I end up doing is this;
Lets say I am playing as Argentina.
1)Save your current game and copy it into your multiplayer folder.
2) Go in and "Host" a multiplayer game and select a major power like China. China always has about 300 PT boats that never move and it never uses parked in The Yellow Sea.
3) Temporarily turn off the initiatives under your SecDef to "None" as well as under "Global Rules of Engagement."
4) Select as many of those boats as you want, give the order for them to be "Scrapped" and send them back to port. China is not going to miss 300 PT boats.
I also will go into reserve units and China normally has around 4-500 AA guns. Go in and delete a bunch of them. Scrap them.
5) Save your game, exit and go back into MP but choose the country you are playing, save it again. Exit.
6) Go into your MP save folder and remove the last save you did and put that into your single game folder. Select that game and you will be back in control of your nation and things should go faster.
If you do this for two or three of the major powers, you should see a decent spike in speed of gameplay for awhile. Some of those units will be replaced but not to the levels they were. Just keep two important items in mind. Don't forget to return the initiative levels back to where they were.
AND most importantly, try to spend as little game time doing this as you can b/c remember, while you're China, the AI is playing your country. So, the more time that elapses the more the AI will change, like selling units that you don't want sold. Or screwing up your production ques. That p***es me off to no end. But most of what you need to do can be done with very little elapsed time.
Another option you can try, if it doesn't confuse you, is for those units you want to deleted. Highlight all of them together and just change their RO&E and make sure you turn "Minister Control" to no. That way, you can send those units back to barracks, or ports and leave and they will still be deleted when you resume your game as your nation.
I know this sounds like a lot, but its really not. I can leave my country and go elsewhere, do what i want to do and return in 2 game hrs usually. In that short of time the damage the AI can do is usually minimal. Of course, I also learned how to change the scenario and remove many of those excess units before I even started the game. If you keep it to just PT boats, AA guns and some artillery, you're not really hurting the other nation b/c the AI does not use those type units as it should or like we do. So, they never really miss them.
Chances are also that you will only have to do this once, at least if the volatility is set high as natural warfare will eventually do the same thing for you.
I hope this is clear enough for you but if ya got a question, feel free to ask!
EDIT: And I did this for a PC that isn't really slow. A friend of mine and myself actually custom built it to play another, much higher end graphics game but for whatever reason, SRCW lags like a mofo by year five or so. Deleting the unneeded units will go a long way to freeing up speed.