what's your screen resolution at work?
If it's 1024x768 @ 24 bpp, 2.25 megs of those 8 megs of v-ram are already used up by Windows. Also, Windows may use double-buffering so it's up to 5 megs that might be eaten up by windows already, unless your app runs in exclusive (full-screen mode).

I don't have an explanation for why SystemMemory := true makes i run faster on your work computer though. Just wanted to point out that 8 megs of Video RAM doesn't necessarily mean "8 megs of usable video ram".