Late last year the new Ukrainian sea drone "Cossack Mamai" was announced. I'm not saying this is what was definitely used in the Ivanovets attack, but it's supposedly capable of a top speed approaching 70mph so would be faster than anything else in the Black Sea.I'd wager that most of those drones were doing 25 knots or better.
5 of those simultaneously bearing down on a ship would stretch their defences somewhat. Even if conditions greatly limited they speed at the time of the attack.
It seems from various reports that Ukraine is now reaching very high volumes of production of all drone types, expecially FPVs.
Traditional Artillery shortages might become irrelevant soon. Short and medium range targets (and a few long range) will be smothered by drone swarms.
Long range and larger targets will be taken care of by (hopefully) continuing supplies of HIMARS, ATACMS, GLSDB, Storm Shadow and I hope, eventually, Taurus missiles.
If UKR can make enough sea drones over the next few months, the Black Sea Fleet may not exist by the end of this year.