Sliding change of speed. Sliding Time Scale/Pitch Shift does the job, but it’s slow.
Time Tracks are fast: http://manual.audacityteam.org/man/time_tracks.html
I forgot about them! It’s inconvenient and inconsistent that sliding speed change is not an Effect, though.
There’s also the optional “Sliding Speed Change” plug-in: http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Sliding_Speed_Change
It is a step backward from Time (no logarithmic interpolation, just like in Sliding Time Scale/Pitch Shift) and it still lacks the convenience of having semitone inputs.
You’re not funny, Captain Obvious.