I picked the one which claims x300 speedup.

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-22-20178

"In comparison,we compiled the CPU implementation, tMCimg,
using the “-O3” option with the gcc compiler and double-
precision computation on a single core of an Intel 64bit Xeon
processor of 1.86GHz."

So single core, low frequency, double precision, out-of-the
box compilation. Enough said.