Spark running very slow cause some requests

[#1]

I use Spark2 on Yarn. I using ORC Files with partitions and caching in memory. I testing concurrent request of Spark.

If just only user request, Spark very fast, take 2-3s to finish.

But when I increase in few user request, Spark seem very slow, It take 10-15s to finish request.

So I need to be able to handle many more requests.

Anyone have any document or practical about this.

[#2]

anyone can help me ?