LVM Snapshot confusion

LVM snapshots are block-level snapshots. So whether you mount them or not, they will have the same impact on the performance of your system

They are, as you guessed, copy-on-write snapshots, so they will never change. When a block changes, a copy is made. This also means that initially a snapshot takes up no addditional space, but as time progresses, space used for keeping the snapshot will grow until the snapshot is destroyed (unmounting does not destroy a snapshot).

Also caused by the copy-on-write is the performance hit caused by all this extra copying (writing a single byte may cause an entire block to be read and copied). Depending on your application's behavior, this performance hit may be acceptable or not, or not even visible at all.