AWS Batch, Real Time rendering - decrease tasks' "Pending" time

Question

TL;DR: My tasks take 4 seconds, but there is 10 seconds delay between submission and execution and I need to decrease that. I suspect tasks spend too much time in "Pending" state on EC2.

We need to have a rendering farm ready 24/7 for our website, so we can serve users a personalized animation in seconds. I have setup an AWS Batch environment (see architecture screenshot) with both spot and on-demand clusters, and there are always ec2 instances running in on-demand. The tasks themselves don't take long, however I noticed a long "start up" time between task submitted and actually started. In the screenshots you can see that the task it self took 4s but the whole process about 13s. I think it is might be related to the fact that tasks spend a lot of time in "Pending" state and I have no idea what that means or does. Is there a way how I could speed it up?