It depends on what you mean by speed as there are several definitions!
We fly the aircraft based on what we see on the 'indicated' airspeed instrument - this measures speed by sensing how much air pressure (due to forward motion) is entering the instrument and converting it to an airspeed.
The higher you go (e.g. 5000 feet versus 100 feet as you quote), the less air there is and the lower the atmospheric pressure. This essentially means you need to travel quicker through the air to get the same reading on the above instrument. So to get the same indicated airspeed (say 140 knots) at 100 feet takes less ACTUAL forward speed through the air (termed 'true airspeed) than getting 140 knots indicated airspeed at 5000 feet (where the air is thinner).
So, for a given indicated airspeed (which the aircraft is always flown on as this determines lift), the true airspeed is higher at higher altitudes i.e. your dial will read 140 knots (if that's the aircraft's landing speed that day) at 100 feet and 5000 feet but, at 5000 feet, you'll actually be travelling quicker through the air.
All of the above is complicated by another definition - 'groundspeed', the speed you're travelling relative to the ground. This is wind dependent.
We landed yesterday with about 5 knots of tailwind - we usually land with a headwind, which is usually at least 5 knots, say. 5 knots of headwind to 5 knots of tailwind represents a 10 kts difference in ground speed and let me me tell you - you notice it and the landing feels a loooot quicker!
If you look up the definitions of indicated, true and ground speed, I think it will all become a bit clearer.
A simplified answer but hopefully a starting point?
B&S
