The Benchmarking says that MMA10 is faster - at least on my machine (MacBook Pro, 2.8GHz i7, 16GB RAM, SSD).
Needs["Benchmarking`"]
BenchmarkReport[]
gives

for Mathematica 9 and

for Mathematica 10. That means that the score went up by more or less 20%, which constitutes quite a nice speed gain. In version 9 have noticed that the same task is quite a bit slower on a Mac than on a Linux machine - I do not have access to Windows. So the speed increase could be due to an optimisation for OSX. We also benchmarked to more Macs, and a standard iMac got a speed gain of about 40%.
Here's the data in another, more detailed form:
{"MachineName" -> "marcos-mbp", "System" -> "Mac OS X x86 (64-bit)",
"BenchmarkName" -> "MathematicaMark9", "FullVersionNumber" -> "9.0.1",
"Date" -> "July 11, 2014", "BenchmarkResult" -> 1.101, "TotalTime" ->
12.568, "Results" -> {{"Data Fitting", 0.493}, {"Digits of Pi", 0.65}, {"Discrete Fourier Transform", 0.82}, {"Eigenvalues of a Matrix", 0.665}, {"Elementary Functions", 1.141}, {"Gamma
Function", 0.549}, {"Large Integer Multiplication", 0.552},
{"Matrix Arithmetic", 1.016}, {"Matrix Multiplication", 1.377},
{"Matrix Transpose", 0.802}, {"Numerical Integration", 0.663},
{"Polynomial Expansion", 0.119}, {"Random Number Sort", 1.561},
{"Singular Value Decomposition", 0.998}, {"Solving a Linear
System", 1.162}}}
and
{"MachineName" -> "marcos-mbp", "System" -> "Mac OS X x86 (64-bit)",
"BenchmarkName" -> "WolframMark", "FullVersionNumber" -> "10.0.0",
"Date" -> "July 11, 2014", "BenchmarkResult" -> 1.236, "TotalTime" ->
11.197, "Results" -> {{"Data Fitting", 0.862}, {"Digits of Pi", 0.444}, {"Discrete Fourier Transform", 0.567}, {"Eigenvalues of a Matrix", 0.572}, {"Elementary Functions", 0.843}, {"Gamma
Function", 0.585}, {"Large Integer Multiplication", 0.466},
{"Matrix Arithmetic", 2.012}, {"Matrix Multiplication", 0.616},
{"Matrix Transpose", 0.808}, {"Numerical Integration", 0.735},
{"Polynomial Expansion", 0.147}, {"Random Number Sort", 1.164},
{"Singular Value Decomposition", 0.78}, {"Solving a Linear
System", 0.596}}}
We can plot this:

If this is correct Matrix arithmetic became slower, from 1.016 in MMA9 to 2.012 in MMA10. Many other functions became faster.
Cheers,
M.