a) IP (Level 3) connectivity
b) Ethernet (Level 2) connectivity
c) Encryption
d) Authentication
Different features create different overheads. e.g. encryption adds overhead but if the path between the two sites is over the public internet that overhead may be necessary.
ipip is the easyest it doesn’t use encription, and do not load cpu. I think you must add route to your lan’s, because routers knows about LAN’s but hosts not. Just add route to 192.168.1.0/24 gateway is ipip’s interface address. You need to do it on the both routers.
Hope it will help!