Not going to lie, but I haven't LAN'd that game in almost 20 years. At that point I was running a Pentium 2 on a 100m netgear switch.
I would give ether one of these a try.
In the RA2 folder find the ra2(md).ini and add this line under video;
[Video]
VideoBackBuffer=no
Info:
game lag is literally all in the game code and in the way is handles rendering objects on the screen. The software wants to render instead of using DirectDraw but DD tries to do it anyway and bogs the game down with unnecessary actions. This modified DLL has a high success rate of reducing lag on most configurations. However, it's not 100% since a few machines here and there experience a black screen with audio and mouse input but no video. That means it does not work on your configuration. However, I am completely unsure of the success rate on Windows Vista but I know it's higher on Windows XP of both flavors.
Turn off hardware acceleration (http://www.pixelmetrics.com/Tips/VidBlank/...cceleration.php)
Info: Some RA2/YR players may experience dramatic in-game slowdowns during special effects, ie:
- Structures blowing up
- Prism tanks firing
- Using super weapons
This is due to outdated game code, which makes incorrect assumptions about modern video cards. Disabling hardware acceleration may negatively affect overall performance somewhat, but will also drastically reduce - if not completely eliminate - these slowdowns. Try running RA2/YR with and without this setting, and leave it at whichever works best for you.