Let's take a look at one WordPress plugin -- it's not the only one that offers this functionality however it's the one I use and we can compare page load performance when the plugin is disabled and then enabled. The plugin in this example is the a3 Lazy Load plugin for WordPress, which is used to delay the loading of images and videos, specifically.
The problem, in this case, is that pages that contain videos and images are going to be potentially very large -- if we can delay loading videos and images until a later time, then the size of the request will drop considerably. Let's take a look at some test results prior to adding the a3 Lazy Load plugin, below.
In this image we can see that the number of requests is at 102, the fully loaded bytes in is at 2,366KB and the hint that this can be improved is that the cost is high ($$$$$) -- if we lazy load the images and videos, this should improve, which can be seen in the following image, which includes test results with the a3 Lazy Load plugin enabled.
In this example we can see that the number of requests has dropped from 102 to 83 (a 19% improvement), the bytes in has dropped from 2,366KB to 491KB (a 79% improvement), and the cost has dropped from $$$$$ to $ (an 80% improvement) -- so overall a considerable improvement, one which should help in terms of improving search engine optimization (SEO) for the entire website.
From SEO Best Practices in :
"Google has indicated site speed (and as a result, page speed) is one of the signals used by its algorithm to rank pages. And research has shown that Google might be specifically measuring time to first byte as when it considers page speed. In addition, a slow page speed means that search engines can crawl fewer pages using their allocated crawl budget, and this could negatively affect your indexation." 
Note that according to , Bing does not consider page load time to be a ranking factor.
Page speed, or lack thereof, can impact user experience as well. It should be obvious that longer page load times will be directly correlated with a higher bounce rate, less time spent on that page, and lower conversion rates. In "Infographic: Web Performance Impacts Conversion Rates" (#4) we can see just one example of where slow page load time can directly impact the user experience as one in four, or 25%, of visitors will abandon a website which takes more than four seconds to load.
This is even more important if your business makes money online -- for example, in  we can see that an increase of just one second in page load time could cost Amazon 1.6bn$ in sales over the course of a year.
Another example comes from the article entitled "Impact of slow page load time on website performance"  -- here we can see that "[f]or every 100 ms of improvement, [Walmart] grew incremental revenue by up to 1%" and seeing that Walmart, including Walmart subsidiaries, was positioned to capture around four percent of online retail spending in the United States by year-end 2018 this equates to around 21bn$ -- so that 100ms in page load time is worth significant money .
The links below are worth reading if you're new to technical SEO and are also interested in the impact that this can have on your bottom line.
If you're wondering what you should aim for in terms of speed, take a look at the article entitled "How to Find Your Website's Performance Poverty Line"  for a more detailed discussion of this topic. As fast as possible is a simple answer but being able to frame that around some expectations based on visitor data should prove to be helpful. Note that the simple answer may, in fact, be a very expensive answer because at some point delivering further performance improvements becomes difficult to achieve and if the return-on-investment remains mostly unchanged, it's simply not worth pursuing.
- Moz Page Speed
- How Loading Time Affects Your Bottom Line
- Google Speed Update: How Fast Page Speed Should Be For SEO
- Infographic: Web Performance Impacts Conversion Rates
- How Page Load Time Affects Conversion Rates: 12 Case Studies [Infographic]
- Impact of slow page load time on website performance
- Walmart Passes Apple to become No. 3 Online Retailer in U.S.
- How to Find Your Website's Performance Poverty Line
- SEO Cheat Sheet #2: Page Speed & Image Optimization
- Assess Your Website Performance in under Ten Minutes
- SEO Beyond Google: How to get 35% more organic traffic from Bing & Yahoo