I'll be presenting some more details at the UKUUG LISA conference in Edinburgh in 2 days. Just a quick note here...
SQLite 18.104.22.168 (unmodified) took ~22.43 seconds to insert 1000 randomly-ordered records on my laptop (with Samsung SSD). The SQLite modified to use MDB took only 1.06 seconds.
This 20:1 improvement in write performance is most significant in terms of absolute efficiency. Given how pervasively SQLite is used in Android and apps like Firefox, on smartphones, tablets, and other battery-powered devices, there's a potential to significantly increase the runtimes of these devices by switching to SQLite+MDB.
Read efficiency is somewhat of a lost cause, >95% of the CPU time in reads is spent in the SQL parser. Even though MDB is much more efficient at reads than stock SQLite, the difference is lost in the noise under the parser overhead.
I believe, for the vast majority of situations where SQLite is embedded in applications, significant efficiency gains would be obtained by using a pure binary data interface. While a simple key-value interface may be too primitive for app convenience, an inline implementation of the X.500 data model (with indexing) may be more useful. I.e., slapd back-mdb turned into an application library.