When I say slow, I don't mean just clock time. But, how two languages were conceived.
In R, all variables are stored in the namespace. In Python, the story is different,
# Python exmaple
> a = [1, 2, 3]
> b = a
> b[0] = 10
> a
[10, 2, 3]
It's passing the address of a to b. Changing b will change a. You can look up soft copy vs. hard copy.
Another example, it's ok to initialize vectors in R like this,
# R example
x <- y <- z <- c()
In Python, this is passing addresses again.
R wins in my book, it's more intuitive. But, when your a is, say, 1GB, what will you do?