My take is, it is not about speed or size but the kinds of operation you want to do to your data.
If you plan to do join, sort, and other DB operations on your data then go for Sqlite. An example is sorting data by date.
If you want to map simple values (like int, boolean, String) then use Preferences. DB operations won't work here and needless to say you need to have all the keys. An example is user password or app configuration.
The big temptation to embrace Preferences is when you want to use it to store a flattened POJO (a serialized JSON object) as String. Having such need is actually the sign to use Sqlite. Why ? Because complex data will eventually need complex oprations. Imagine retrieving a specific entry which could be handled by a simple "SELECT ... WHERE id = 1". In Preferences path, this will be a long process from deserializing to iterating the results.
