Are there anyone who can give some tips on how to fixe these problems? I am not an expert in page speed optimization in any way.
I would recomad you to run the Google PageSpeed Insights https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fbluvi.dk%2F for a better overview of your network issues.
When the browser loads the page, for every script found, it will have to download, parse and execute the script. This behavior is render blocking, nothing gets rendered until those steps are done. Setting a script async will execute that script asynchronously as soon as it is available and will not block the initial render.
Specify image dimensions
This does not always work as images needs to be responsive and often do not have the same sizes on various screen resolutions.
Remove query strings from static resources.
That will not be possible as Shopify has the query strings built in for every asset on the CDN.
Do you use a lot of apps and plugins on your site? Hopefully they use async js already. Some js needs to load with the page, others don't, so it depends a lot on your site. The process of doing it isn't simple - you have to find js includes (they look like <script src="long url"></script> in different places in your theme files), and adding the "async" part could break certain page elements. If you're going to try it, I'd recommend making a copy of your theme and making changes to the copy, then setting it live when you get it working.
You can also use https://tinypng.com to optimize your images for free.
Does your site load slowly? An app like GTmetrics might always find ways to improve your page load speed, but maybe it's listing low priority issues because your theme, apps, and images are already pretty well optimized.
Page speed is pretty important. Google applies speed to its ranking algorithm, and pages that take 3+ seconds to load could already be leading to visitors leaving.
If you are using wordpress, there are many plugins available to increase the page speed like Autoptimize plugin. It can aggregate, minify and cache scripts and styles, inject CSS in the page head by default but can also inline critical CSS and defer the aggregated full CSS, moves, and defers scripts to the footer and minifies HTML. It can also optimize lazy-load images, google fonts, remove Wordpress core cruft, and more. So, one plugin can solve many problems and helps to increase your website speed easily.
In SEO perspective, page speed is one of the most important ranking factors. Keeping your site speed up is all about boosting user experience and accessibility. To improve the speed of your Shopify store, you could take into consideration using a fast, mobile-friendly theme, using smaller and optimized images, using Shopify speed optimization apps, and avoiding the use of sliders.