Looking through the initial requirements I would really like to schedule a call to talk over some of these items in details.
I understand your needing to convert your site to using the WooCommerce system for the Product Vendor plugin, and also need to be able to support the other common elements of WooCommerce like Shipping, Orders, Commissions, etc.
I will go over each element in kind to demonstrate I understand what you are looking for and provide a couple of paths to get there:
1. Page Speed Optimization
We will want to get a baseline to start from, and work towards a set objective. My suggestion is to start with a measurement for speeds on 3G(Mobile), DSL Speed, and Cable/Fiber Connected speeds. Then we will need to define the finishing point. Looks like 80/100 is your goal based on Google's PageSpeed insight score.
- In order to get there here are the typical steps sites need to take:
-- Optimize custom code, custom themes, and images on the sites
-- Optionally Lazy-Load content that is below the fold in order to get the page to initially show up faster
-- switch to a headless solution, this is the more complex solution but would reduce your load time drastically (Freelancer.com is an example of this)
2. Integrating WooCommerce Vendor Marketplace.
Estimating how long this will take to do, depends mostly on the custom code ....
Running out of space lets schedule a call to finish this discussion.