From Jeremy:
In the discussion section at the end from Landin's next 700 PLs, Strachey says
"One inconvenient thing about a purely imperative language is that you have to specify far too much sequencing. For example, if you wish to do a matrix multiplication, you have to do n^3 multiplications. If you write an ordinary program to do this, you have to specify the exact sequence which they are all to be done. Actually, it doesn't matter in what order you do the multiplications so long as you add them together in the right groups. Thus the ordinary sort of imperative language imposes much too much sequencing, which makes it very difficult to rearrange if you want to make things more efficient."