@miketica
Hi There, From my experience the heavy part of performances issue in WP is JS, CSS and images. Need to minify , bundle and remove from header. The other slowdown issue is the numbers of requests from server. even if its small images , the time to request X http from server can slow down the page dramatically.
The download speed on pingdom should be less than 1 min on a good VPS server. Your site have 1:30 sec which is not bad , but you have 99 requests which is too much this should go down to 30 -40 requests as max.
https://tools.pingdom.com/#5ad9221b20000000
On google you can find more issues with static images for cache that can be help a lot
https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fprogressivetree.com%2F&tab=mobile
Start with your mobile responsive and that will effect the desktop as well.
If you need my help , please let me know
Tomer
