Docker for NAS devices is the de facto App Store. The most compelling applications are for media consumption / piracy.
Synology's ARM line is generally underpowered compared to their Intel offerings, in terms of clock speed, memory capacity, and core count. They also tend to gate additional features (e.g. hardware-accelerated encryption) behind their Intel offerings.
edit: We're mostly talking Atoms / Celerons here, so not exactly screaming performance either way.
There are third-party package managers for the ARM devices but there are a lot of broken packages so I wouldn't recommend using it.