1-No way will you see real-time performance of AVCHD on virtually any computer. SR5 is AVCHD, not HDV.
2-Running Vegas in Preview/Auto mode should provide realtime internal and external preview of HDV to SD monitor or HD LCD/plasma monitor
3-Kill all background apps, particularly antivirus, and you should see an immediate increase in playback performance.
4-HDD’s set to DMA and not PIO.
5-Be sure HDD’s aren’t completely/nearly completely filled.