A few points to increase efficiency :
[Other than needing more space beneath the spawner and more water sources (which can be generated from the first two placed) there are no extra resoureces needed.
These changes also make it easier to upgrade the farm to inlcude a 'softener' and/or the option to convert the mobs to drowned/strays.]

With the 'new' rules only allowing mobs to spawn at light level zero a single torch placed either atop or on any side of the spawner is sufficient to fully spawn proof the entire 9x3x9 spawner spawning area. [​This can be useful if using the accessway to de-light the spawner. (see at bottom)]

per the wiki "If 6 or more mobs of the monster spawner's type have their hitbox intersecting a 9×9×9 volume centered on the monster spawner block in Java Edition… … the monster spawner "poofs"; without creating any mobs and then waits for the next cycle"
This means one must keep the top of a mob's hit box greater than 4 below the level of the spawner: given zombies & skeletons are ~2 high and 'bob' somewhat in water, the collection floor should be 7 below the spawner (7 air blocks between the spawner bottom and the floor {or 6 air and 1 water}).
[​Note how this is calculated has changed over the years, check the information for the specific version. (Older versions counted the mobs position as the location of its feet so 6 was sufficient.)]

The 1x1 central drop can also delay mobs making to the kill site as the mobs can bump each other preventing entering the drop. (One can see this at ~0m10s in the video).
A better tactic is to have an 8-wide water floor carry the mobs to a 1 wide, 2 deep trench along one side of the spawn room; this trench then carries the mobs to the drop.
(e.g. water flows from west to east across the main floor and from north to south in the trench with the drop at the south end of the trench.)
[If one estblishes the player kill spot directly below (2-deep) the drop (i.e. not using an elevator/ drop shaft or other 'softener') the player's feet will be 12 below the spawner. (7 to the main floor + 2 to the trench floor + ~2 to the hopper {level with the player's feet + 1 for the player's feet}).
The light level in the space above the hopper should be 10 or less to keep every space in the 9x3x9 spawn volume at zero]
The critical distance is that the player's feet should be <15.5 from the spawner [ SQR(Δx² + Δy² + Δz² ) .

Particularly for zombies (which have a 1-high variant), two glass rather than a bottom slab directly above the spawner is recommended. [​It is marginally possible for a mob that spawns 'in' the 'transparent' slab to exit up rather than sideways.]

Something that does require extra work (more mining) is to create an accessway to the spawn room such that the player can reach the spawner from an opening in one of the walls.
When the farm is constructed one can then break the torch on the side of the spawner and seal the access in comparative safety. (The torch is even recovered via the water streams :) )
This access can be used to shut down the spawner if one wished to make upgrades at a later date.