Its not so much lagging as when you have the setting at 900ms the chart is only refreshed every 900ms. So sometimes it will be right on time ... but equally often it will be 900ms late. On average though the data was received 450ms before you saw it on the display so I guess it feels like lagging. I think 900 is only really suited to end of day or hourly bar type traders; half a second won't matter much in that case.
I must admit I hadn't realised that it could be as fast as 50ms now so I must update mine to 100ms (recently got an 8500 to replace my e6600 and added an extra 2G of ram so why not speed up sc too
)