If this were the primary reason, it seems odd that some of the earliest popular languages used 1-based indexing in an era with much slower CPUs (<= 1 MIPS) and less available memory to store instruction code. If the authors of FORTRAN were comfortable with 1-indexing on the IBM 704 (40 KIPS) in 1957, it couldn't have been prohibitive. At the same time, as computer use cases became more interactive there may have been a greater focus on performance, but it likely that the semantics of the high-level language were at least as important.