It kinda depends on if images and videos are encrypted separately and only indexed at first.

How much data there are on your chats? 1 megabyte is around one thick book in plaintext.

AES-CBC as example method decrypts more than 2 gigabits per second with hardware opcodes (2012 processor), for example if we look this data https://www.bearssl.org/speed.html

It is impossible to say based on delay when searching plaintext on this level whether there is encryption.