"confirm or deny" is not possible, because it shows you assume that there is a black or white answer, which there is not. this also shows how you do not know max/m4l, as if you did you'd realise that it is all dependent on what you are programming with it and also, how you do it. this is one of the things the writer of the text you posted gets wrong. it is more complicated than that.
shreds of truth: maybe in november 2009 when m4l was at it's greenest, there were performance issues, but even so, the computer your quoted user was using must have been very very slow (5% idle?!?! - in november 2009 i had a completely awful falling apart very slow shitty computer and even i got better usage than that). nowadays, m4l performance is excellent. of course there is a performance 'hit' to live - you are encompassing an entire complex software environment inside another. running in the live thread is the amazing thing about m4l - you can patch realtime and compile real time. there are big problems with copying and pasting devices though- this is still slow. on their radar apparently.
"All M4L devices run on the same core" is not really true / partly true. one m4l device can only access one core (unlike the advanced multithreading options available in max standalone) but that is true of all 'tracks' in all 'daws' - if audio is processing it must reside in the same environment. lives still spreads its total workload across available resources automatically. it keeps some away from m4l so it can continue to run smoothly.
"I will not touch on M4L again until they fix this" is a bit like the kids that clamour for 64-bit but do not really have a clue what it would actually mean and the way they use their computer would probably make no difference to them. however, maybe that not fair of me - it just so happened that i have always wanted what m4l has to offer, warts and all, and the writer did not, which is fine of course. each their own.
there are many wishes for m4l, and ableton and cycling 74 will most probably deliver most of them in the future and surprise us with others. however, as it stands it is an environment for building your own completely unique processing routines that no-one else would do, not an environment for making a 2048-voice fft-granular synth that can drive a car on the side. although max6 looks pretty good for that...
also, it is just down to how it is approached / used / thought about. i do many complex and 'me-type' things in m4l, my current resources are modest at best. it is the best thing that ever happened to live. the writer of the text you quote has a few valid points, but you have to ask yourself why you might care?
then again, i'm one of the ones who never really had any stability problems with live 8x apart from around just when m4l was first released, and could never understand the huge issues of others. so you could always safely ignore me.
phew. that was long. not meant to be a rant - just bored and friendly.
in short.. we live in disappointing times..