I was trying to get more information so that the specialists can join. So it is about a single EC2 instance on AWS, which serves four Moodle instances (through Apache virtual hosts?) but the fifth one is very slow. AFAIC this is AWC specific. I don't know how much expertise is available in this forum. You could try the Large scale Moodle deployment group https://moodle.org/mod/forum/discuss.php?d=414040.