Six years later...
This can be caused by overheating.
Laptops tend to collect dust over time, which clogs their airways, their tiny fan, and the fins of the CPU heat sink. Also,
The heat-conducting paste between the CPU and its heat sink tends to deteriorate over time, (especially if the CPU is subjected to extreme heat a few times,) and the result is, again, insufficient extraction of heat from the CPU.
These conditions cause the CPU to heat up to extreme temperatures.
Now, every modern CPU has heat sensors and associated circuitry which causes the CPU to throttle down its clock in order to avoid burning the chip. You see, modern digital electronics only consume power during clock transitions, so the lower the frequency of the clock, the less frequently it transitions, so, the less power is consumed.
(Incidentally, a lower clock rate can also accommodate a lower voltage for even greater power savings, and this is a common power-saving strategy in modern systems, but I think that when the CPU self-throttles its clock the voltage is not reduced, because the voltage regulation is done by the motherboard, which is blissfully unaware of the stress that the CPU is undergoing.)
Anyhow, the apparent effect of this throttling down of the CPU clock is extremely slow performance. It is like having a CPU which executes a few instructions, (clock cycles,) heats up, stops executing anything, waits to cool down, and then proceeds to execute a few more instructions, all in a cycle of less than a millisecond. You see, the CPU die is a very small piece of metal and silicon, so it has a very small thermal capacity, so it heats up very fast, and it also cools down very fast.
In order to determine whether overheating is actually your problem, you need to obtain some program which measures the CPU temperature over time, and see where it stands. Generally, the 60°C/140°F ballpark is okay, while the 80°C/180°F ballpark is too hot, although the specifics of your system may differ.
If overheating is your problem, the first and easiest thing to try is to have the airways of the laptop cleaned from dust. This might work, but there is a considerable chance that it won't work, because your laptop has already been subjected to extreme heat, so the heat-conducting paste may have been damaged. So, if cleaning the airways does not work, then the next thing to try is to have the heat-conducting paste replaced. And since this is a laptop, this should only be done by a qualified professional.
I am not a qualified professional, but I have replaced the heat-conducting paste on two different desktop computers which were exhibiting very poor performance for no apparent reason, and lo and behold, the problem was completely fixed in both cases.