It's only slow when you open since it needs to extract some files/binary. But it's size shouldn't be changed at all since Electrum (AppImage version) still store it's data on ~/.electrum.
AppImages are for novices. I said what I said.
They are convenient in some situations, and give simplified options for people who aren't technically capable, which is fine, but I tend to find them limiting. I suppose it's not a huge deal for Electrum since most of the things you can do with CLI commands can be done in the console.
I wouldn't use the Debian repository for Electrum, and we should discourage newbies from doing so also. The repository is always several versions behind, which could (and has in the past) pose security risks.