Should a 3m cable charge speed be significantly different than a 1m cable? I recently bought a 3m (meter) Apple charging cable and found that it was charging my iPad slower than usual. 

I plugged it to a USB tester and found that it charged at ~5v 1A.  By comparison, my 1m cable charged at around 5v 2A. 

Is it this normal, should there be that much of a difference?

I'm testing by charging an iPad. The wall adapter is the genuine 12W iPad wall outlet rated at 5.2v at 2.4A. I have a genuine Apple 1m cable and I also bought a 3m and 1m cable third party cable off eBay. They are unbranded.


Both genuine and non-genuine 1m cables charged my iPad at around 5v 2A. 
The non-genuine third party 3m, cable charged the same iPad at a bit less than 5v 1A.


The cable nor USB tester gets warms up enough to feel, but the wall plug does, so I don't think it will be (hope it won't be) a fire hazard.

Here are some pictures of the USB tester, maybe that will help. First one is a with the 3m cable 



This one for the 1m cable
The 3m (meter) 3rd party cable is the problem here.  There are two possibilities:


it's for an older iDevice that didn't have or support quick charging (iPods, iPhones, etc.)
it's a cheap knockoff where the manufacturer didn't supply the correct chip to request higher power delivery


When selecting a 3rd party (non-genuine) Apple accessory, it's always good to go with known brands - I myself prefer Anker or even Amazon basics.

Understanding power adapters and cables

There's a lot of misunderstanding in the question about the role cables play in the charging process.  Let's clear some of that up...


The cable itself, doesn't dictate how much power drawn.  Two cables of similar AWG (gauge or diameter of the wire) isn't going to draw vastly different amounts of power.  For the inherent resistance of the cable to become a factor in dropping power, we start looking at distances in the kilometer range (a thousand times longer than what you're working with).
A typical 20 AWG copper cable can support from 4A (multi-core like braided wire) and up to 6A (solid core) of current.  At 2A, you are well within the tolerances and "getting warm to the touch" is perfectly normal.
There are many differences for the amount of current draw on a device:


Load -  an idle computer draws less power than one crunching away displaying video
Charging state - a fully charged battery draws less power to charge than a depleted one.  Also intelligent chargers go from "rapid charge" to "trickle charge" once a threshold has been reached.
Power Negotiation - intelligent devices like quality Apple cables or USB 3.1 chargers negotiate the amount of power delivered.  



The issue specific to your situation is the last point - power negotiation.  In Apple cables - both Apple branded and high quality 3rd party - there is a chip that negotiates with the power brick how much power to deliver


Photo courtesy Chipworks

If the chip in the cable only requests 1A of power, the power brick will only deliver 1A of power.