On my tests, doing a very simple example (100.000 times storing and retrieving a string on the Couchbase server) i saw major performance difference from SDK 2.7.
The .Net SDK v3.0.7 is used against a Couchbase server 6.6
The .Net SDK v2.7 is used against a Couchbase server 6.0
The performance decrease was about 40%.
For the storing / retrieval of string using the .Net SDK v3 i used RawStringTranscoder which is, as the docs say, the fastest to store simple strings
Actually i did also other tests with other object types (for example dictionaries) with the same results, always performance degradation.
Is there any performance benchark done internally for the performance either of new SDK or new Couchbase server? A 40% decrease is quite huge.