1: Unsubstantiated and vague statements, e.g. X is 50% faster than Y, well, how is it measured, what are the actual numbers?
2: Measuring performance by measuring how much time a program takes, e.g. X takes 3 seconds while Y takes 6 seconds, well, is that consistent across measurements, have you measured it on different machines, have you measured that under various different conditions?
Various factors can impact the apparent performance of code, among which is memory layout. Considering on GNU/Linux (and maybe other platforms) the global environment is passed into a program by putting it above the stack (remember, stack grows down) the memory layout can differ by virtue of having different values for the environment variables USER (username), HOST (hostname), and PWD (current working directory); apparent performance is not really reliable.
I don't know of the best practices for measuring performance, but if I were responsible for measuring the performance of a program I'd do so by looking at its disassembly, assigning a cost to the different types of instructions, and adding the costs together. That probably is a terrible way to go about it, so I welcome any insight into why that isn't done.