Increasing num of indices decreasing performance

Elastic Stack Elasticsearch
1

I have one index per user. With one index i would be able to achieve 5k docs per second. Each doc ~5K. When i increased number of users and hence number of indices... around 30 users the num of docs per second reduced to 700 per sec.

Is there any settings which help in tuning? should i increase the index thread pool size?

2

How many nodes do you have? Why not one single index with filtered aliases per user instead?