A device implementing adaptive, cross-device event data synchronization includes at least one processor to maintain a database on the device, the database storing event data items derived from user data items corresponding to user activity on the device or a second device. The at least one processor periodically transmits, to a server, first insertion records corresponding to new event data items generated at the device, the transmission periodicity based on a frequency at which the new event data items are generated. The at least one processor, upon detecting deletion of one user data item, deletes, from the database, one corresponding event data item, and immediately transmits, to the server for propagation to the second device, a first deletion record for deletion of one corresponding event data item at the second device, the first deletion record being transmitted along with any of the first insertion records awaiting transmission.