I'm using [feed to post RSS Aggregator] and get posts from 44 sites. main page loading is too slow. I checked GTmetrix and it seems good but I don't know how can I improve my website loading. http://akhbartop.ir/
Loading RSS feeds over network is extremely slow operation. WP's
fetch_feed() has built-in caching for it, but it won't help much if one page has to check feeds in number of dozens.
This is likely unsolvable without fetching feeds completely asynchronously from page load (not something WP can natively do) and without plugin you use making proper use of that.
Though guessing the performance bottleneck is rarely certain. You should figure out what takes time in your case specifically. There is range of tools for that from WP plugins (such as Query Monitor) to proper PHP profilers (such as Blackfire).