Very slow transfers to/from micro SD card on a RPi B+

[Jean-Baka Domelevo Entfellner <domelevo%gmail.com@localhost>]

Hi there,

I'm currently untarring a pkgsrc.tar.gz (48MB) to /usr. The
commandline I used is:
# nohup tar xfz pkgsrc.tar.gz -C /usr &

so nothing is written to the stdout, it justs gunzips and untars. And
I have the feeling it's extremely slow. I'm running:
NetBSD rpi 7.0_RC1 NetBSD 7.0_RC1 (RPI.201507072130Z) evbarm
and it's on a RaspberryPi model B+

The microSD card is an ADATA (32GB), I don't think it's class 10,
maybe class 4 (can't remove it from its slot now while the RPi is
running). The boot messages concerning it are:
ld0 at sdmmc0: <0x03:0x5344:SS32G:0x80:0x2554f92d:0x0e4>
ld0: 30436 MB, 7729 cyl, 128 head, 63 sec, 512 bytes/sect x 62333952 sectors
ld0: 4-bit width, bus clock 50.000 MHz

So, while the untarring command is running, I check the speed with iostat:

      tty              ld0             CPU
 tin tout  KB/t  t/s  MB/s  us ni sy in id
   0   20 7.767   15 0.113   0  0  1  3 95
   0  131 8.194   31 0.246   0  0  3  3 94
   0   44 7.871   31 0.236   0  0  1  3 96
   0   44 7.793   29 0.219   1  0  1  2 96
   0   44 8.133   30 0.236   0  0  2  2 96
   0   44 7.871   31 0.236   0  0  1  1 98
   0   44 7.793   29 0.219   0  0  1  4 95
   0   44 7.939   33 0.253   0  0  2  1 97
   0   44 7.931   29 0.222   0  0  0  2 98
   0   44 7.571   28 0.205   0  0  6  4 90
   0   44 7.793   29 0.219   0  0  3  2 95
   0   44 8.294   34 0.273   0  0  3  5 92
   0   44 8.000   28 0.217   0  0  3  0 97
   0   44 8.067   30 0.234   0  0  0  3 97

It's on average between 0.20 and 0.25 MB/s, don't you think it's
unreasonably slow? Do you have any piece of advice for me?

Thanks,
   JB