They can’t fire you after just one advertisement, but it should not be taken lightly either. What do you mean by non-pick times? If your picking rate for preparing orders is lower than the average rate why is this? Are you not familiar enough with the store layout? If you only do 2 days a week do they change things in between times and maybe arriving 15 minutes before your for shift and walking round the aisles to get a feel for the changes would be beneficial, or do you just walk slowly from one product to the next whilst your colleagues run?
As for your breaks, if they really are too long then that’s easy to correct.
Go to the meeting and listen to what they have to say without interrupting. If you agree with what they are saying ( you’ll know deep down if they are telling the truth and you haven’t been pulling your weight) then apologise and tell them what you will do to improve. If however, you think they are being unfair, then state your case politely. If you are aggressive they’ll just wait for the next opportunity to call you in and your job really will be on the line. They’ll have no trouble replacing you so best to eat humble pie.