[Onetouch Idol 3 General] petrov.0:
Here are some test results:

for i in `busybox seq 1 5` ; do time dd if=/dev/block/bootdevice/by-name/boot of=/stoarge/sdcard1/bootimg$i ; done ; busybox md5sum /storage/sdacrd1/bootimg*

65536+0 records in
65536+0 records out
33554432 bytes transferred in 8.343 secs (4021866 bytes/sec)
0m8.39s real 0m0.50s user 0m3.54s system
65536+0 records in
65536+0 records out
33554432 bytes transferred in 8.568 secs (3916250 bytes/sec)
0m8.66s real 0m0.38s user 0m3.87s system
65536+0 records in
65536+0 records out
33554432 bytes transferred in 8.695 secs (3859049 bytes/sec)
0m8.75s real 0m0.33s user 0m4.03s system
65536+0 records in
65536+0 records out
33554432 bytes transferred in 8.488 secs (3953161 bytes/sec)
0m8.55s real 0m0.56s user 0m3.70s system
65536+0 records in
65536+0 records out
33554432 bytes transferred in 8.756 secs (3832164 bytes/sec)
0m8.83s real 0m0.46s user 0m3.98s system
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg1
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg2
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg3
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg4
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg5

for i in `busybox seq 1 5` ; do time dd if=/dev/block/bootdevice/by-name/boot of=/stoarge/sdcard1/bootimg$i bs=4096 ; done ; busybox md5sum /storage/sdacrd1/bootimg*

8192+0 records in
8192+0 records out
33554432 bytes transferred in 2.032 secs (16513007 bytes/sec)
0m2.36s real 0m0.07s user 0m0.65s system
8192+0 records in
8192+0 records out
33554432 bytes transferred in 2.095 secs (16016435 bytes/sec)
0m2.20s real 0m0.03s user 0m0.63s system
8192+0 records in
8192+0 records out
33554432 bytes transferred in 1.971 secs (17024064 bytes/sec)
0m2.08s real 0m0.02s user 0m0.53s system
8192+0 records in
8192+0 records out
33554432 bytes transferred in 1.909 secs (17576968 bytes/sec)
0m2.31s real 0m0.05s user 0m0.48s system
8192+0 records in
8192+0 records out
33554432 bytes transferred in 1.861 secs (18030323 bytes/sec)
0m1.98s real 0m0.04s user 0m0.53s system
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg1
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg2
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg3
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg4
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg5

for i in `busybox seq 1 5` ; do time dd if=/dev/block/bootdevice/by-name/boot of=/stoarge/sdcard1/bootimg$i bs=8192 ; done ; busybox md5sum /storage/sdacrd1/bootimg*

4096+0 records in
4096+0 records out
33554432 bytes transferred in 1.208 secs (27776847 bytes/sec)
0m1.53s real 0m0.00s user 0m0.34s system
4096+0 records in
4096+0 records out
33554432 bytes transferred in 1.431 secs (23448240 bytes/sec)
0m1.48s real 0m0.04s user 0m0.30s system
4096+0 records in
4096+0 records out
33554432 bytes transferred in 1.465 secs (22904049 bytes/sec)
0m2.43s real 0m0.01s user 0m0.35s system
4096+0 records in
4096+0 records out
33554432 bytes transferred in 1.440 secs (23301688 bytes/sec)
0m2.34s real 0m0.02s user 0m0.35s system
4096+0 records in
4096+0 records out
33554432 bytes transferred in 1.443 secs (23253244 bytes/sec)
0m1.52s real 0m0.05s user 0m0.40s system
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg1
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg2
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg3
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg4
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg5

for i in `busybox seq 1 5` ; do time dd if=/dev/block/bootdevice/by-name/boot of=/stoarge/sdcard1/bootimg$i bs=16384 ; done ; busybox md5sum /storage/sdacrd1/bootimg*

2048+0 records in
2048+0 records out
33554432 bytes transferred in 1.057 secs (31744968 bytes/sec)
0m1.37s real 0m0.02s user 0m0.29s system
2048+0 records in
2048+0 records out
33554432 bytes transferred in 1.435 secs (23382879 bytes/sec)
0m1.54s real 0m0.01s user 0m0.32s system
2048+0 records in
2048+0 records out
33554432 bytes transferred in 1.666 secs (20140715 bytes/sec)
0m1.89s real 0m0.00s user 0m0.36s system
2048+0 records in
2048+0 records out
33554432 bytes transferred in 1.316 secs (25497288 bytes/sec)
0m2.32s real 0m0.01s user 0m0.33s system
2048+0 records in
2048+0 records out
33554432 bytes transferred in 1.389 secs (24157258 bytes/sec)
0m1.48s real 0m0.01s user 0m0.32s system
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg1
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg2
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg3
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg4
51f9ceb989f85b55843096a7c9a5602a /storage/sdcard1/bootimg5

From them the fastest operation seems to be with bs=16384. It is possible to have better times above 16384. But I think that this is enough.