On many flight simulators, I have noticed that planes tends to get slower with increasing altitude. For example, I can reach 1100 knots just above the sea level in Google Earth flight simulator(F16), but only about 400 knots at 11km with the same plane.
This is because what you are looking at is the IAS indicator (Indicated Air Speed). This represents the amount of relative air which flows over and under the wings of the plane. This is what creates lift and enables the plane to fly. This is why this instrument is so important and belongs to the primary flight instruments.
This is not to be confused with GS (Ground Speed), which can be sensed by IRS (Intertial Reference System), or by the GPS. Ground Speed is not relevant to pilot an aircraft, but it's still useful for example to determine the ETA (Estimated Time of Arrival), and for fuel estimation during flight.
The IAS indicator indicates a lower speed because the more you climb, the less air there is. So even the air flows faster, the global pressure feeling is lower.
So, actually planes are not slower at higher altitudes. They are faster (at least most planes whose flight envelope force them to fly faster when they are higher, otherwise they would stall, because they would have to increase their Angle of Attack too much to counterbalance the lack of air flowing above and under the wings). But when they reach their target Mach speed, their speed will decrease slowly as the outside air temperature is decreasing until end of climb.
Sorry for the picture being in French... But as you can see, the flight envelope of an airliner forces him to actually fly faster as it is climbing.
There are different speeds on an aircraft, and it can be quite confusing.
So, why is the primary airspeed indication (IAS or CAS) the wrong speed (not actual speed) of the air flowing around the aircraft? This is because this wrong airspeed represents actually the real pressure feeling around the wings that is responsible for the lift. It is this parameter that we need as pilot and it is used in aircraft manuals to describe the V-speeds. It's perfect that IAS and CAS are not corrected with altitude and temperature, because if we know that our airplane stalls at 120kts for example, we won't need to care about altitude or temperature. Our airplane will stall at that same speed regardless of altitude and temperature. If we would use TAS, stall speed would be constantly changing and it would be more difficult to keep the aircraft in good and safe flying conditions.
Here is where you can find these speeds in the Boeing 737 NG cockpit:
What you actually see in the cockpit is Indicated AirSpeed (IAS). The airspeed indicator actually measures dynamic pressure (which depends on speed and air density). The instrument assumes that density is 1.225 kg/cubic meter, regardless of altitude. In real life this doesn't happen. As you go higher the density decreases and so does dynamic pressure. The instrument feels dynamic pressure decreasing and because "it knows" density it's constant it will show you that airspeed is decreasing.
During the climb there is a point where the climb is continued by maintaining a fixed Mach number. This point during the climb is called the cross over altitude.