bash: read stdin from pipe into variable

x.sh

% printf '%d\n' {1..10000} | { time a=$(</dev/stdin) ; echo "$a"; } | wc -c

real    0m0.027s
user    0m0.004s
sys 0m0.004s
48894

% printf '%d\n' {1..10000} | { time read -rd '' a; echo "$a"; } | wc -c

real    0m0.041s
user    0m0.004s
sys 0m0.016s
48894