If you expand the "leave a gap of 2 seconds" rule to instead follow the car in front of you by 3 seconds, then someone cutting in should only require you to add 1.5 seconds of distance to the new, smaller gap. And since the car cutting in will often tailgate the car in front of you, it's often really around 750ms you'd have to bleed off, which shouldn't require much speed change.

We're not really talking about that, we're talking about stop-and-go traffic, and smoothing out the stop-and-go portion through the use of an expanding and shrinking buffer in front. It will by necessity be larger and smaller at different times.

Additionally, when the gap is lessening because the car in front it stopped or at the slowest speed of the variable speed traffic, another vehicle inserting itself into this buffer can make you slow rapidly, thus negating the point of leaving a buffer to smooth out the variable traffic speed.

Sometimes I also cut off such interlopers. I can accelerate quite rapidly in first. After a while, surrounding drivers learn to respect that space in front of me.