I am seeing very little diffference between versions 9 and 10 on 64 bit platforms. Likewise on the 32 bit machine I checked.

To elaborate on the point made by Ilian, enlarging the size of the integers to the 32-64 bit range shows a markedly different behavior.

Clear[data];Clear[data];
now = SessionTime[];
abs = AbsoluteTiming[
   tim = Timing[
      data = Reap[
         Do[Sow[Partition[Range[i^2]^2,i]], {i, 1000}]
         ];
      ];
   ];
ver = StringTake[$Version, StringPosition[$Version, ")"][[1, 1]]];
Print[ver, " Session=", SessionTime[] - now, " Absolute=", abs[[1]], 
  " Time=", tim[[1]]];
ByteCount[data]
Clear[data

]

In versions 9 and 10 this takes around 6 seconds. In version 8 it takes 120 seconds.

Not a misprint; I tested all three versions on the same machine. Notice that I changed the problem so as to use integers larger than 32 bit but smaller than 64. This means that version 8 cannot work with packed arrays whereas the later versions can do so.

My investigation started with an M9 program that I benchmarked and published on RosettaCode.org. After I installed M10 (and uninstalled M9 in the process) the program was much slower. The facts are clear. M10 is slower. M10 needs a fix. If you have a 32 bit kernel go ahead and test.

"In versions 9 and 10 this takes around 6 seconds. In version 8 it takes 120 seconds."

Is that a misprint or are you referring to M8 on a very slow machine?