Is Tracking Slowing Down my Website?

Is Tracking Slowing Down my Website? 

We employ various techniques such as image compression, server-side caching, JavaScript and CSS bundling that can make an enormous difference to page load time. After a fair bit of digging we often discover that this isn’t a specific concern but is based on the output of a tool. These tools, Lighthouse for example, are very powerful when used for SEO but don’t really tell you the whole picture. It is also vital to note that these tools do not understand any context of your business or your website and they are not actually your customers using your website. 

Running Lighthouse against a well-known online retailer shows a few recommended optimisations that can be made including deferring offscreen images (which is loading images you can’t yet see but probably will later after the page has loaded and a banner has shown the next slide), serving images in next-gen formats (which at the time of writing not all browsers support so is a bit of an odd recommendation) and so forth. 
One thing it doesn’t say is: remove tracking code or ads.
I’ve run this against quite a few websites and it never says that. Not once. I have also run other similar tools and they don’t recommend any removal of these either. 

This must mean that all the other tracking tools are fine to use and don’t slow your website down at all. 
That is not the case - they can slow down your page load. Even if only a little bit. 
I have run a little experiment on a website that while it has no adverts, it does have tracking code so this experiment will only be affected by tracking code. I have disabled caching and used a simulated Fast 3G network. Then I will compare the time to load the page with ad blocking disabled and enabled.

With ad blocking enabled the page is loaded in about 7 seconds (repeated requests showed about the same)

With ad blocking disabled… about 9 seconds (repeated requests showed about the same)

This is obviously a very small snapshot and the experimental conditions adjusted to highlight the problem. If you simulate more typical conditions the times seem to be 3 seconds with ad blocker disabled and 2 seconds with it enabled.
Does that second make a difference? It probably won’t make that much difference to your SEO but it won’t complain. It won’t make much difference if your customers are on a fast connection but it will help. It will make a difference to your customer if they are using a slow internet connection such as a slightly dodgy 3/4G signal. 

I’m not saying you should remove your tracking code this instant as it can provide very useful information when configured well. You are interested in what products people look at, what they add to a basket, what they purchase and so forth but how many different systems are you sending it to? Are there any systems that you were using but are no longer using? 
Our e-commerce platform does store quite a bit of this information already and we are always looking for ways to improve our product so will always be adding extra metrics (if you have any ideas then do let us know).
You could be sending information to 3rd parties unnecessarily and do you know for sure what they are doing with your data?
In conclusion:
- check to see what your e-commerce platform already stores to see if you even need to add tracking
- if you are going to use a tracking tool then use one (and only one!)
- take care with personal customer data you are sharing with 3rd parties.