I am currently deciding between a 940xm laptop and a portable desktop (clevo 7200 with a i7 950) so I gathered some data from Toms benchmarks. I could only find benches for the 920xm, the 940xm is very similar, only a few % difference, basically anything multi-threaded the 950 trounces it, even the 930 is 40%+ faster than the 920xm, but anything single threaded, and the 920xm is 30% faster than a desktop 950, and 10% faster than a i7-975 Extreme.
test suite i7-950 920xm % increase
3d mark vantage CPU 19252 12631 52%
Sandra 2009 ALU 64.4 41.8 54%
Int/Float 120 77.7 54%
Mem band 23.3 16.8 39%
MainConcept 1.6.1 MPG2 to 264 1.45 2.23 -35%
iTUnes Audio CD 1.58 1.45 2 threads 9%
TMPGEnc 2.12 3.03 -30%
Lame 2.43 1.90 1 thread 28%
winzip 4.67 4.62 1 thread 1%
3ds max 0.43 0.65 -33%
AVG AntiVirus 2.98 3.07 -3%
note times above are in decimal, not minutes:seconds
sources
http://www.tomshardware.com/reviews/core-i7-975,2318-5.html
http://www.tomshardware.com/reviews/mobile-core-i7,2443-7.html