Compiler mis-recognizes contiguous reinterpreted matrices?

I am implementing an ODE to use with OrdinaryDiffEq. I am simultaneously evolving in time multiple quantities, which are combinations of vectors and matrices, some real and some complex. The approach I took has been to group them all in a long Vector{Float64} and and my function f!(du, u, p ,t) which computes derivates, I slice into that long vector. However, I noticed that this leads to poor matrix-matrix multiplication performance on the obtained slices for some methods of slicing. I tracked d...