Yes, your blog posts won't show up on the Portfolio index page because that is supposed to show portfolio posts, and converting a page to Portfolio Layout is not really the solution for speeding up your site performance.
You can do some of the performance tips provided here instead.
Like I said above your images are already optimized (they load fast), what the GTMetrix means to "Serve scaled images" is to resize your images to somewhere (378px by 173px) using a photo editor (e.g. Photoshop) and re-upload it to your site. There is one trick that might resolve this but most likely reduce the quality of your images in other areas of your site (not recommended). That is to reduce the Settings > Media > Large size, and use this plugin to regenerate the images with the updated size.
I checked your Posts Grid Settings and you have the Total Items as 99999 technically that will load all your posts, and yeah that takes time. Try to reduce that, somewhere 20-40 items.
Always clear your caching plugins for the changes to take effect immediately.
Your site loads fast on my end, so this could also be something on your local network, have you tried checking your site outside your local network and see how fast/slow it loads?