The IPSEC overhead is not that bad, assuming you have enough bandwidth. As far as the latency, once the VPN is actually setup,it is roughly about the same as as without encryption
VoIP + IPSec Bandwidth Calculation
Configuration / Transform set options
IP GRE (Generic Routing Encapsulation)
IPSec Tunnel Mode
ESP (Encapsulating Security Protocol)
Encryption 3DES (esp-3des)
Authentication SHA-1 (esp-sha-hmac)
Layer 3 Packet Size
G.729 = 136 bytes @ 50 pps = 54,400 bps
G.711 = 280 bytes @ 50 pps = 112,000 bps
Layer 2 (FR / HDLC) Packet Size (+4 bytes)
G.729 = 140 bytes @ 50 pps = 56,000 bps
G.711 = 284 bytes @ 50 pps = 113,600 bps
