$ for i in {10,100,1000,10000,100000}; do dd if=/dev/urandom of=test.txt bs=4096 count=${i}; done
10+0 records in
10+0 records out
40960 bytes (41 kB) copied, 0.00423367 s, 9.7 MB/s
100+0 records in
100+0 records out
409600 bytes (410 kB) copied, 0.0391987 s, 10.4 MB/s
1000+0 records in
1000+0 records out
4096000 bytes (4.1 MB) copied, 0.36739 s, 11.1 MB/s
10000+0 records in
10000+0 records out
40960000 bytes (41 MB) copied, 3.66702 s, 11.2 MB/s
100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 38.7499 s, 10.6 MB/s