Last week the checkin for a 17.40 departure opened at 15.10. Two and a half hours before departure.
by this time there was a substantial queue in both the Club and regular checkin desks who, like myself, assumed it would open three hours before.

Istanbul is beautiful airport, but be prepared for unfriendly staff. The Havas agent checking me in did not say a single word or acknowledged me when checking in at the Club Europe checkin. No eye contact, just throwing my passport and boarding pass back at me. Same at the gate, and security. You can spend a fortune on the airport, but no amount of money is going to change the service!

Allow for THREE security checks. The first to enter the terminal where all your luggage is scanned. The second after passport control. The third at the gate which is a manual search, and a frisk.
Finally, it appears that BA doesn’t pay for fast track passport/security clearance as I was denied. Luckily the airport was quiet at the time, so there was no wait in the other lines.
Once you are airside, there is an abundance of shopping and food options.