ALG - Application Level Gateway
PPTP - Point to Point VPN = disable no problem
IPSec (VPN) - Client VPN - disable no problem
SIP - VoIP - disable no problem
Disabling the above will not mean you can't have VoIP or a VPN, it just makes it harder to setup.
The ALG is to allow this traffic to passthrough and the sessions to be monitored and controlled correctly - if there's no device / client on th lan requiring it you turn it off.
If won't help with performance though. ALG on our router takes up 8 bytes of memory
ALG is sort of like a pre-defined port forward (not quite) - it's a security featurehttp://en.wikipedia.org/wiki/Application-level_gateway