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.

The Mathematica installation includes both 32-bit and 64-bit kernels, so one can try both. This particular operation is as fast in the 32-bit V10 kernel as it was in V8.

Obviously there is some tradeoff, but 64-bit integers allow for computations with much larger arrays and expressions, up to any amount of installed memory that were simply inconceivable before.

The above comparison with V8 performance is as expected: 64-bits integers are twice as heavy as 32-bit integers. However, it does not explain why your program would run slower with V10 than V9.

Hello all, how can I run a notebook created in mathematica 8, in a win7 64bit pc with mathematica 10.3, in 32 bit and not 64 bit? M10 seems indeed much slower than M8.

Thank you in advance

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.