Anyway I thought snap was merely the name of the package manager, why are they slow on start up then?
snap is indeed a package manager but snap-packages are not regular packages like .deb or .rpm
snap packages are a kind of containers, so although they run on the same kernel as other packages, they have their own libraries, filesystem and environment and share as little as possible with the host system. This is because of security and this is why they are slow to startup (they have to load a lot more than the application only).
You could say that they are complete mini-systems which are 'sandboxed'
On filesystem level, a snap is effectively an isolated filesystem that is mounted at boot time as a virtual device via the 'loop' devicemount.
try this in a terminal:
df -h
and see that all your snaps are mounted as filesystems 
There seems to be a trend within ubuntu to switch internetbrowsers over to snap packages. Not only because of the fast security update cycle of browsers but also because of the added security that these sandboxes deliver.