EDIT: @a3nm's comment prompted me to review the post-mortem of the last conference we helped organise.
The scientific panel met to consider about 1,500 submissions. It took them 2.5 months to reach a decision. The decisions were entered into a database by the secretariat. This took two days because the members of the panel wanted hard copies. The spreadsheet was double checked by the chair of the panel. It took eleven days for him to get back to us. During this time, we were busy with timetabling, which we actually completed in four days. The letters were sent out two days after we received the chair's confirmation. Contrary to my response below (which I wanted to preserve to demonstrate my own error and bias), the main delay we experienced wasn't to do with timetabling.
1,474 submissions for presentations
355 accepted: 120 oral, 235 posters
1,119 submission rejected
These were the results for the last conference I helped organise. Now imagine having to send the correct letter to all 1,500 people. For the 355 people accepted, the letter needs to contain details such as the date, time and place of their presentations or poster displays.
We needed a team of 9 professional staff to assist us, including three wholly focused on timetabling and venue management.
I'm impressed that they managed to inform you in four days.