In reply to Rick Wicklin.

In R it barely makes a difference using the following code ( a similar thing happens when you specify cv and rv to be matrices):

n <- 300
s <- 20000
a <- runif(n^2)
dim(a) <- c(n,n)
a <- a + t(a)
cv <- runif(n)

t.row <- system.time(
for(i in 1:s) p <- cv %*% a
)

t.col <- system.time(
for(i in 1:s) q <- a %*% cv
)

t.row;t.col;