Need Help with Plugins and a Custom theme

Hello, Little background of myself, I am a template designer and have used other CMS programs, and now I am trying to use WP, reason being is WP has lots of plugins.
I am a custom template designer, so any CMS I use I start to learn how to design custom theme/templates ONLY. I do not want to work with pre-made templates. Just wanted to make that clear.

I an in need of help though, I feel like I got WP figured out, I have created my first template and I works great. It’s the most basic template you can think of, index.php and a style.css

Plugins installed never had any errors…

I have recently split the index.php into header.php and footer.php
(just to let you know I have tried this)
But my template worked fine with basic index and style

So here is the Issue
I am experiencing slow performance (transition) on three plugins that I have tried, for example the “responsive menu” plugin which is a plugin I really like that will simplify the mobile menu part of my template, but it is SLOW.. I do not understand what I am doing wrong, I have basic template files. As soon as I open my mobile menu on my Android Chrome browser takes like 3 seconds for it to pop up.
Other two plugins are Meta slider and Shift Nav.
Shift Nav same problem.
And if I add a meta slideshow to the template while my shift nav or responsive menu plugins are working, the performance slows down so much, it is unusable.

The slider I have also noticed that it is slow transition.

As soon as I activate a different theme, everything is butter smooth.

I am hoping someone can just tell me what am I missing from the basic setup that I have. Here it is..

theme/images
theme/index.php
theme/style.css
theme/header.php
theme/footer.php

Nothing complex no added jquery, just a few google fonts

I understand I am not very descriptive, I feel like this is a simple problem, but if needed I would like “you” to tell me what “you” need to answer my question. If you do need more info.
But please don’t redirect me to troubleshooting links, MY Template WORKS fine and I know and already have troubleshooted the basics as well as compared my theme to a working theme. I feel like I am just missing a wordpress function/file that is causing all this.

Did I forget to include a common WP Tag or plugin or template file?

This is what I need
I am trying to receive an answer from an experienced custom template designer, because I know for a fact that they have bumped into this when they started off.