Why is generating a higher amount of random data much slower?

I want to generate a high amount of random numbers. I wrote the following bash command (note that I am using cat here for demonstrational purposes; in my real use case, I am piping the numbers into a process):

for i in {1..99999999}; do echo -e "$(cat /dev/urandom | tr -dc '0-9' | fold -w 5 | head -n 1)"; done | cat

The numbers are printed at a very low rate. However, if I generate a smaller amount, it is much faster:

for i in {1..9999}; do echo -e "$(cat /dev/urandom | tr -dc '0-9' | fold -w 5 | head -n 1)"; done | cat

Note that the only difference is 9999 instead of 99999999.

Why is this? Is the data buffered somewhere? Is there a way to optimize this, so that the random numbers are piped/streamed into cat immediately?