rain@mws64:/tmp$ for i in {1..5000000} ; do echo string $i >> file ; done
rain@mws64:/tmp$ sed -n '1234567p' filbash: fork: Невозможно выделить память
<перезапустил шелл, ибо память почему-то осталась занята>
rain@mws64:/tmp$ time sed -n '4234567p' file 
string 4234567

real 0m0.700s
user 0m0.627s
sys 0m0.062s

rain@mws64:/tmp$ time (head -n4234567 file | tail -n1)
string 4234567

real 0m0.298s
user 0m0.269s
sys 0m0.155s

rain@mws64:/tmp$ cp file file1
rain@mws64:/tmp$ cat file1 >> file
rain@mws64:/tmp$ cat file1 >> file
rain@mws64:/tmp$ wc -l file
15000000 file

rain@mws64:/tmp$ time sed -n '14234567p' file
string 4234567

real 0m1.776s
user 0m1.629s
sys 0m0.137s
rain@mws64:/tmp$ time (head -n14234567 file | tail -n1)
string 4234567

real 0m0.867s
user 0m0.866s
sys 0m0.411s

rain@mws64:/tmp$ time (tail -n$((15000000+1-14234567)) file | head -n1)
string 4234567

real 0m0.064s
user 0m0.053s
sys 0m0.012s