Comment 19 for bug 947664

I'm experiencing this problem as well. I suggest all of those with the problem post some information to help narrow the issue down. In my case I'm using a newly installed SSD and have a feeling the slow linux-headers unpacking is just a side effect of slow write speeds.

Hard drive: SSD (Corsair CSSD-V60GB2 w/ firmware update)
Size: 60GB
Filesystem: ext4 with dm_crypt full-disk encryption

Ubuntu 12.04 LTS
Kernel: 3.2.0-36-generic

Approximate time to unpack: 30 min

# hdparm -tT /dev/sda2
/dev/sda2:
 Timing cached reads: 6526 MB in 2.00 seconds = 3270.42 MB/sec
 Timing buffered disk reads: 398 MB in 3.00 seconds = 132.57 MB/sec

# dd if=/dev/zero of=tempfile bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 16.0206 s, 67.0 MB/s

During the delay:
iHR linux-headers-3.2.0-37-generic 3.2.0-37.58

Immediately after the delay:
iU linux-headers-3.2.0-37-generic 3.2.0-37.58

linux-headers is unpacking 8152 small files, so my hunch is that there is nothing special about the package, nor a problem with dpkg, but instead that linux-headers is simply demonstrating the already slow disk speeds we have but don't notice with other packages.