Proto3 is not a large change. In fact it shares most of its code with proto2, as I understand it. The underlying encoding is the same.
Proto3 removes some features from proto2 which were deemed overcomplicated relative to their value (unknown field retention, non-zero default values, extensions, required fields) and adds some features that people have wanted for a long time (maps). But all of these features are things that are "on top" of the core, not really fundamental changes.
I think the only change which affects my comparison post (linked by GP) is removal of unknown field retention. This is actually noted in the comparison grid. I'm honestly very surprised that they chose to remove this feature since it is critical to many parts of Google's infrastructure.
I don't think any lookup table is provided (the wire order of entries is undefined). They are not lookup maps, they are syntactic sugar for repeated key/value pairs.
The "lookup table" is constructed at parse time. Yes, the items are just key/values on the wire, but when parsed (in C++, at least) they are placed in a google::protobuf::Map, which is hashtable-based. I guess it could differ across languages -- some might not have any specific support for maps yet.
Proto3 removes some features from proto2 which were deemed overcomplicated relative to their value (unknown field retention, non-zero default values, extensions, required fields) and adds some features that people have wanted for a long time (maps). But all of these features are things that are "on top" of the core, not really fundamental changes.
I think the only change which affects my comparison post (linked by GP) is removal of unknown field retention. This is actually noted in the comparison grid. I'm honestly very surprised that they chose to remove this feature since it is critical to many parts of Google's infrastructure.