I mean it’s not news that Cyengine in many ways is further ahead in the graphics side of things (but backwards in other areas) compared to UE4 but the statement “unoptimized as it can be” is a tiny bit of an exaggeration obviously 
I get the point Maximum-Dev is trying to make though, it can be frustrating sometimes when another engine has that “one” better feature that UE4 doesn’t have (or is way worse at) but it goes both ways… try to use Cryengine for a few months and you will miss plenty of UE4 features, and especially the community.
But anyways, enough with the engine comparisons… that never ends anyway :rolleyes:
To answer the original post…
15 trees sounds too little to be affecting performance in such major way unless you are using high poly trees without LODs (or…?).
You should profile your scene and/or turn dynamic shadows on/off etc. to figure out what is actually causing the perf. drop.
Then from there you can try various techniques to optimize your scene… you can look into imposter sprites, hierarchical LODs and even roll your own system (not using the foliage tool).