spring cloud zuul gateway taking long time to serve requests, best practices to tune it

Question

I am using spring-cloud-netflix-zuul for our gateway for multiple services, when I observed the performance of each application using appdynamics I found that the request is taking 2 - 23 ms at gateway and the specific service to process the request but from gateway to service the HTTP is taking 250 - 500 ms so just wondering why it is taking long time. I have gone through the document found here and I have increased the per host max connections and total max connection and I see we are using SEMAPHORE and the performance results were good, it got improved but I

Steps Taken:

1. Increased the maxConnections zuul.host.max-total-connections = 2000
2. Increased maxConnections per host zuul.host.max-per-route-connections = 500
3. With Default semaphores
4. Increased semaphores - zuul.semaphore.maxSemaphores = 3000

Observations:

1. With increased total and per host max connections found the HTTP time got decreased
2. With default or increased or decreased to 1 semaphore didn't see any difference

Questions:

1. Is there any queue to check/debug when max connections per host is say 10 and gateway got 1000 requests where these 990 connections sits?
2. When to increase semaphore how it is useful?
3. How to calculate per host connections to set Max Connections per host and also total max connections?
4. How to check the IsolationStrategy of my gateway? I see it as "ribbonIsolationStrategy": "SEMAPHORE" but I am not using ribbon/serviceId concept, I am using host concept where I provide all zuul.routes with host,path,url.