Installation and upgrades on btrfs are *extremely* slow. This is because dpkg makes extensive fsync() calls to make sure that a power loss in the middle of an operation does not leave the system in a broken state. These calls have a significant performance penalty on other filesystems ( such as ext ), but on btrfs, the penalty is multiple orders of magnitude.
As a workaround, you can use the eatmydata package/command around dpkg/apt-get to disable the fsync() calls and restore good performance during upgrades, at the risk of hosing the system if it crashes. The apt-btrfs-snapshot package will have apt make a snapshot before it begins an upgrade so that if things do go wrong, you can at least roll back to the snapshot.