i suspect what you see with the 2G is the extra round trip delay. both 3G (UMTS), and now 4G (LTE) have decreased the latency vs 2G. I suspect that opera mobile is not applying any kind of pipelining (basically reusing a single http connection to send multiple page elements), especially when turbo is enabled, and so each element of the page that needs to be downloaded is its own http connection. Each connection needs to be negotiated and each will be affected by the extra latency of 2G compared to 3G.

heck, it may even be beneficial to disable turbo under such conditions if it can lead to use of pipelining and therefor decrease the need to set up connections.

but this is all speculation.