I suggest you read this Performance Tutorials - Google PageSpeed & Webpagetest.org as to know how Google Pagespeed scores for specific metrics and you shouldn’t focus on the score number but individual metric values themselves. FYI, your mobile pagespeed score isn’t entirely related to images.
From your site’s webpagetest.org test results at https://www.webpagetest.org/result/191120_9T_dd74efd6776e67bb580488488fd762a6/3/details/#waterfall_view_step1, it’s pretty clear slowness is due to your origin server/webapp when index page takes 2.922 seconds to load where TTFB is 2.79 seconds. This matches what your PageSpeed Insights’ mobile/desktop First Contentful Paint (FCP) metrics for both lab and field (CRuX Chrome User Report real world visitor data) report for 2.7 and 2.8 seconds respectively. You will never get faster FCP metrics if your average TTFB for origin response time is anywhere close to 2.79 seconds on 5Mbps cable speed tests let alone for 3G mobile 1.6Mbps speeds tests.
You didn’t mention the type of web hosting you using ? shared, VPS, dedicated server ? You may be able to get better origin performance with better web hosting. For instance not all VPS hosting is equal. I benchmark tested 13x VPS servers from Upcloud vs DigitalOcean vs Vultr vs Linode vs Hetzner and the performance differs drastically in some cases !
Magento is known for it’s slowness unless you do optimise Magento at server origin side. No Cloudflare optimisation will help as CF doesn’t cache dynamic index HTML generated content by default https://support.cloudflare.com/hc/en-us/articles/200172516#h_a01982d4-d5b6-4744-bb9b-a71da62c160a.
dom interactive time was 3.99 seconds and this is on cable 5mbp speed test. On 3G mobile speed test and 1.6Mbps download, it sure will be slow - giving you poor time to interactive numbers on mobile and given it has highest weight in Google PageSpeed Insights v5 scoring formula = low mobile score.
And your webpagetest’s Google Lighthouse metrics https://www.webpagetest.org/lighthouse.php?test=191120_9T_dd74efd6776e67bb580488488fd762a6&run=3
But yes as folks touched on image sizes need to be optimized at origin (images you upload). I have been using free irfanview software for over a decade and very useful for optimising images.
Take your 146.5KB slider image for example
at 90% optimisation on right shows size reduced to 71.39KB
at 85% quality optimisation on right shows size reduced to 60.97KB
though using converting to webP would be even smaller for lossy 75% quality webP image. But to serve that from origin, your web server or web app needs to know how to conditionally serve webP to only web browsers that support it and fallback to original image for web browsers that don’t support webP. So Cloudflare’s webP conversion via mirage would be needed on Cloudflare Pro plan or higher.