Ask the Experts / Lag-Free Rain

Over the summer I had to invest in a new PC because the laptop I had been using was deemed irreparable. So I splurged a little bit and built something that might be considered a "gaming PC". (AMD 8320, 16GB RAM, Radeon 7800 HD, 120GB SSD, Windows 8.1, etc.)

I know RCT2 is an old game—ancient by today's standards—but I still experience lag on parks with a lot of detail, or with a lot of peeps. This problem doesn't exist, however, when it rains. During inclement weather, I get a perfect framerate and I can't understand why.

Is there anybody else that has experienced anything like this?