Engineering Explained is back with another helpful video to help us all sound like we know what we're talking about. This time, Jason Fenske took the time to demonstrate the difference between boost threshold and turbo lag.
It's a fairly simple distinction. Boost threshold applies to the actual engine speed you need to achieve before your car generates enough exhaust flow to spool the turbo. There's a complex equation to explain the whole thing, but you'll need to watch the video for that level of detail.
Turbo lag is exactly what it sounds like. It's the delay in time between opening the throttle when over the boost threshold and actually experiencing the surge in power. Fenske gets into much more detail in the video.