I'll concur that IB's data feed and trade execution has no lag that I'm aware of. Their platform is pretty dependable.
I'm not sure of their exact ceiling on the number of real time charts you can have open simultaneously at IB, but it's probably about 50. I had over 40 IB charts open today.
You can reach a point where you try to open another chart and IB will give you a message saying you already have the maximum number of windows open, and that you'll need to close one or more before you can open a new chart. Rarely do I have problems with charts not updating or not working properly. Occasionally, at the start of the trading day, after 9:30AM, it can take a few minutes before charts start updating properly. But this is not a typical occurrence.
I used to have constant problems with IB's charts and platform taking forever to load, and freezing up on me. And I was unable to have more than about 8 charts open and functioning at once, at the best of times -- but it turned out this was NOT the fault of IB. The culprit was my outdated, slow old Dell pc with insufficient RAM! Since I upgraded to a new pc last year, IB's platform works beautifully.