The Linux Logical Volume Manager (LVM) supports creating snapshots of logical volumes (LV) using the device mapper. Device mapper implements snapshots using a copy on write system, so whenever you write to either the source LV or the new snapshot LV, a copy is made first.
So a write to a normal LV is just a write, but a write to a snapshotted LV (or an LV snapshot) involves reading the original data, writing it elsewhere and then writing some metadata about it all.
This quite obviously impacts performance, and due to device mapper having a very basic implementation, it is particularly bad. My tests show synchronous sequential writes to a snapshotted LV are around 90% slower than writes to a normal LV.