The insane lag in GH is caused by the extra fishers, remember that it was playable back in 1.5. Also Giran square is much smaller area, and because Giran is specifically designed to be a trading center, many shops would be spread across the city - crystal sellers around Luxury store, low equip sellers around armor/weapon shop, material sellers from ivy, out middle through the connector to the Taurin, talisman sellers in front of the luxury shop etc., some Dye sellers in Magic shop etc. etc. The initial lagspike would be nonexistent.
I agree that GH is far more comfortable, but then again, you know what else would be comfortable? Free NPC buffer. But the important factor here is that both NPC buffer and custom free teleport to a custom area are insanely anti-classic. People who want comfortable gameplay seek other kinds of servers. Giran has been a trading area for almost 14 years and is an embodiment of a classic Lineage 2.