Why does fan increase slowly its speed at boot and system is so slow? I am given a MacBookAir5,2 [A1466 (EMC 2559)] that is very slow. To give you an idea, a sequence of:


Power on
Log in and loading the desktop
Power off from the menu


takes around 10 minutes.

The MacBookAir runs quite cool. With $ top I see around ~10% of CPU usage after logging in and ~25% of RAM and sysctl hw.cpufrequency returns 1,800,000,000 without the commas, which I suppose is good. Also, I noticed that fan speed is always at (almost?) full speed.
So, after searching around I did a PVRAM/NVRAM reset and an SMC reset.

PVRAM/NVRAM reset seems to be successful since I get the "double restart".

But I question the SMC reset. The steps of the procedure are:


Shut down the computer. 
Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected. (I shut it down having MagSafe plugged in)
On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time. 
Release all the keys and the power button at the same time. (I did this, holding them down for &lt;1, ~5 and ~10 second(s).)
Press the power button to turn on the computer. (Now, this does NOT work. I have to unplug MagSafe in order for power button to turn on the MacBookAir.)
Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC. (It does not change states to me.)


The strange thing is that when I turn it on, the gray screen appears with the "loading" icon. Then, after let's say 10 seconds I can listen to the fan increasing its rotation slowly from its minimum speed to -I guess- the maximum. This increase is quite slow and takes around 10 seconds.

Should the next step be delivering it to a proper technician or there is something else I can do?
Go to iFixit and look up the guide for your Air. You will need to do two things. First is disconnect the battery, second is remove/clean the CPU heatsink before reapplying thermal paste and refitting it. When you've done that you can reconnect the battery and see what happens.

SMC resets using key combos don't always work - disconnecting the battery does. It might seem drastic but removing the power source is what's needed. If you've an SMC issue (which sounds very likely) this should clear it and cleaning up the CPU cooler will give you something useful to do while any residual voltage is discharging.