Modules are items that can be inserted into buildings that have module slots in order to enhance their performance and capabilities in some way. They are also ingredients in making some late game items. There are three different types of Modules, and three levels of each type. The three types are Speed Modules, Efficiency Modules, and Productivity Modules.

Each one does something different, and the Speed and Productivity ones have some downsides as well. What each module does and some examples of good uses for them will be explained below...

Speed Modules: Speed Modules increase the speed of the machine that they are inserted into. This comes at a cost though, because they also make the machine consume more power, and more power means more pollution, at least while you're using Steam Engines for power generation. There are three levels of each Module type, each with increasing benefits and downsides. The effects of each Speed Module are as follows…

Level 1 Level 2 Level 3
Speed Bonus +20% +30% +50%
Energy Consumption +50% +60% +70%

As you can see, they can greatly increase the speed of a machine and the things produced within it, but at the cost of higher energy consumption for that particular machine. So for a quick example, let’s use an Electric Furnace. Normally it takes 180kw of power, and has a smelting speed of 2 items/s. With one level 1 Speed Module inserted into it, the smelting speed will go up to 2.4 item/s but the energy consumption will increase to 270kw.

So in summary, Speed Modules are good to use when you either can't produce something quick enough, or need a lot of something very quickly. Definitely pay attention to the extra energy consumption though because it can go up very quickly if you just go around throwing Speed Modules into everything.

Efficiency Modules: The Efficiency Modules are pretty straight forward. They decrease the energy consumption of the machine they are inserted into. Not only does it decrease the energy consumption, it also lowers the pollution produced by that machine, because energy (at least while using Steam Engines) causes pollution. So by lowering the energy consumption by say 50%, the pollution from the power it uses will decrease by 50%. There is one limit to how low the energy consumption of something can be though. It cannot use less than 20% energy (you can’t reduce it more than 80%), so trying to lower it past that is pointless. The different levels of Efficiency Module is as follows…

Level 1 Level 2 Level 3
Energy Reduction -30% -40% -50%

These modules are best used when you're low on power, and best used on things that either take a lot of energy or that are constantly running. Electric Furnaces, or Electric Mining Drills are some good examples of this.

Productivity Modules: The Productivity Modules add a second progress bar in the machines they are inserted into.

That second progress bar fills up as things are produced from the machine, and once it reaches the end you get an extra item(s). So essentially you can get extra free items from the same amount of input resources, but at the cost of reduced overall speed, and increased pollution from that machine. Productivity Modules are best used when resources are low. Turning Alien Artifacts into Purple Science Packs is a great example of this. The Productivity Modules do have some other limitations. They can only be used for intermediate products. So they can only be put into something that is making intermediate products. Here is a link to the wiki page that lists all the intermediate products in the game.

They can also not be put into beacons, whereas the other two types can. The effects of each Productivity Module are as follows…

Level 1 Level 2 Level 3
Productivity Increase +4% +6% +10%
Speed Decrease -15% -15% -15%
Pollution Increase +30% +40% +50%

So at the cost of speed and the increase of pollution from that machine, you gain some extra items. Let's take Alien Artifacts for example. You get 10 Purple Science Packs per 1 Alien Artifact normally. With one level 3 Productivity Module put into the assembler making the Packs, you get +10% productivity. Which means for each Artifact that is turned into Purple Packs, you get 1/10th of that second progress bar filled. So in other words, for every 10 Alien Artifacts that are turned into Packs, you get an extra one, so in this case an extra 10 Purple Science Packs. This may not seem like a ton, but it does add up and really helps when used for something where the required resources are scarce.

Now obviously the modules can be used in combination with each other if you want. So you could put Speed and Productivity in a machine to offset the speed reduction from the productivity. The energy consumption and pollution produced would obviously be crazy though. Or you could do Speed and Efficiency or Productivity and Efficiency. To conclude, the modules can be used to enhance the performance or abilities of machines. Although it can come at a cost though when using Productivity or Speed Modules, so just keep that mind.