A move is described as slow when it puts too much emphasis on the stability of one's own stones at the expense of development, or if it tries to undermine the stability of opponent's stones, which the opponent can resist and play elsewhere.
Depending of the level of play, a slow move can be a mistake or an affordable choice. Since the AI revolution there is even more emphasis on speed of development. What used to be an affordable choice at the professional level may now be considered slow.
Aftern the joseki in the upper left, White's is a little early in approaching the Black group, as the other corners are still covered with a single stone. For Black to respond with is considered slow. Next, White can play in the lower right corner with an approach or a 3-3 invasion.
Instead a move like is considered fast. Now White can indeed sweep away Black's territory and base with the sequence - but up to Black creates (strong) influence towards the centre. The position is even.
(Incidentally, may be a small mistake in itself. See ...)
(From Now I Would Play This Way: Go Seigen looks back at 18 games (Go Seigen: Omoide no 18 Kyoku: Ima Nara Ko Utsu), game 14, p. 205)
Go Seigen says that "was a little slow". His recommended continuation follows.
Note: KataGo believes that after Black should invade at A or B in the lower left. here is losing 7-8% and about 1 point.
Let me try to explain a bit. The shimari in the bottom left corner does nothing to address Black's nirensei. White's play in the top left corner does, because the normal joseki continuation gives White an extension on the top side, which thwarts Black's normal development from the nirensei. Therefore Black does not play the usual joseki, but extends to the top side, anyway. At this point, the top right corner becomes urgent. (Although Go Seigen does not discuss the play, I expect that the wariuchi on the right would be slow, too.)
was popular among Japanese pros around 1970 and earlier, but went out of favor for being "too slow". Current professional assessment is better informed than it was then, but the move may still make a comeback. It is too early to say that it is too ineffective. However, regardless of its efficacy, the kosumi is slow.