A zombie reinforcements copper farm that can be built in about 90 seconds and can give you thousands of copper ingots per hour. Maximum rates will depend on the performance of your specific PC or server, but several thousand ingots per hour would not be unexpected.

In 1.21+, replace the ceiling block over the chain with a waterlogged leaves block.

IMPORTANT
Your game basically needs to manage two things when the farm is running:
1) Summoning productive zombies that will eventually convert to drowneds to make copper, and
2) Killing excess zombies to throttle the farm so that it doesn't ruin the game.
To maximize the rates for the farm, you want to minimize the killing of excess zombies so the lag from killing those zombies does not take away from making productive ones. Don't create huge spawning spaces for the farm because this just creates a lot of excess zombies that serve no purpose (and they bring excess armor into the farm, which requires more frequent culling). Most players need only the 2 linear spawning zones in cardinal directions.

IanXO4 discord server:   / discord  
WDL with 4 examples of copper farms:
https://www.mediafire.com/file/himk4a...
Hopper-Minecart-Based Collection for 1.19 (by Mirage Wedjak and Freeze):
https://www.mediafire.com/file/6fzpbj...

PRIOR WORK REFERENCES:
Gnembon:    • Boost Your Spawner 600% with This Sim...  
Pl: https://b23.tv/SFOMTQj
MaoGe: https://b23.tv/ExKYte5
Turtle0628 (build at Y64 or higher): https://www.mediafire.com/file/rqf8by...

0:00 Intro
0:36 Material List
1:08 Mechanics
7:07 Build Tutorial
11:39 Stocking the Farm
13:55 Instructions for Use
15:46 Culling the Farm
17:25 Upgrades
18:15 Farms Examples (see WDL) and Outro
19:27 How to move a zombie to the outer end islands

NOTES
1:25 Zombies that convert into drowneds will not drop tridents.
2:34 This carpet script will show the spawn zones when you right click the mob with a stick: /script download utilities/reinforcement_zones.sc
3:20 The sword can be made of any material. It does not need to be diamond.
7:20 Some modified servers have dimension-specific difficulty settings, so the End dimension may be in easy or normal difficulty when other dimensions are hard.
8:27 The spawn zones extend out to a distance of 40 blocks away from the attacked mob, but the actual zones are shorter than 40 blocks long.
9:27 You can use netherrack instead of soul sand for the fire. Soul fire isn't as bright and deals more damage to baby zombies that fall past the chain, but it doesn't really matter.
12:27 The zombie picked up seeds, which will prevent it from despawning. That said, most zombies are not able to pick up items, so I would not rely on this technique.
12:47 Backup zombies can also be trapped using boats placed near the farm.
14:33 Do not use Carpet mod for autoclicking. It's best to use an external autoclicker program, but Tweakeroo may work in many cases when set to 45 gt. If you are not permitted to use an autoclicker, set a metronome to 53 bpm and attack every other beat. If you are on a laggy server, you can try a slightly longer attack interval (such as 2400 - 3000 ms instead of the usual 2250 ms).
16:03 Sharpness conveys through an armor stand with Sweeping Edge, but Smite does not.
17:36 Thanks to Freeze and Mirage Wedjak for designing the hopper minecart collection system for 1.19. The schematic link is above, just below the WDL, which also has a closely related variant of the collection system.
18:05 Crouching down while attacking the armor stand will keep you from picking up XP orbs, which further improves client-side performance since the XP orbs will not need to be rendered. You will need to stand up on occasion to pick up the XP orbs to refresh your sword with Mending, and Unbreaking will allow you to stretch the durability of your sword between Mending sessions.
18:11 Using Tweakeroo to disableEntityRendering will drastically improve client side performance. Reducing render distance to 3 chunks will also help.
18:24 The spawning tunnels need to be 2 blocks wide because reinforcements spawn at the northwest corner of the block (so widen the tunnel to the north or west). Spawnproofing is not needed in the overworld tunnels because skeleton spawns cannot see any targets, so they do not shoot the arrows that could miss and hit a creeper instead, which would explode in retaliation.
18:38 Many baby zombies are ejected from the farm due to cramming if they climb a ladder while inside the farm (since their feet are no longer in a ladder block). You may see them yeeted on occasion.