I agree with the quoted poster, but I do think the situation is a bit of the bell curve/midwit tweet.
The original paper didn't really understand memory-bound kernels, and had some very incorrect assertions on performance.
On the other hand, once you start really pushing performance (like matmul epilogues), the additional state tracking needed for the mean shift makes it somewhat more painful to fuse.
So yes, layernorm is not 2x more expensive than rmsnorm. But it's not "free" either.
every research team needs to spend some time learning how their modeling code lowers down to gpu kernels
example: rms norm vs layernorm
industry assumes rms norm is cheaper. you don't need the x̂, it looks simpler, so it must be faster, so it got adopted
but that's not true. both rms and layernorm are memory bound kernels (the amount of time it takes to get the data to gpu cores is longer than the amount of time it takes to do the computation on said cores)
both take the same amount of time e2e
so can probably train the model right with either, but maybe it makes a difference (e.g why diffusion models still use norms with an affine shift e.g adaLN)
i get why. as open-source models get better and, inevitably, commoditized across the inference providers, overall serving speed determines user experience determines which model gets adopted
but there's no reason to superstitiously avoid free things