On the first issue, this depends. It depends on how high poly the trees are, your quality settings and how many you’re placing.
Speed trees on Terrains usually don’t cause problems at all. Maybe try going into Edit > Project Settings > Quality and changing it from Fantastic, assuming it is. But usually it’s not even an issue on Fantastic if all there is simply speed trees and terrain. The grass of the other side… Loads of that can definitely slow your computer. And, it’s true your computer, by looking at the information, isn’t super high end. But it’s definitely way more than enough to get a bunch of speed trees in there.
Also, inside of the speed tree view there is an option that is checked by default which says “Enable Tree Colliders” with loads of colliders, if all are colliding with the Terrain, it’ll decrease performance even on better computers. Especially if they’re all programmed to do things upon collision. But it shouldn’t hurt Edit Mode while Play Mode is off at all. But just make sure your trees isn’t colliding with eachother or the Terrain. By changing layers. But I’m pretty cure they don’t collide with the terrain. Maybe with themselves, though. Again, no. It shouldn’t decrease performance. (Of course unless your Terrain is at the size of 2000 and your trees are very small causing like 500 different trees with colliders to fit on there a bit spaced out… Then maybe.)
I’d say get lower poly trees. Or just don’t use Mesh colliders on the trees. Box our Capsule Colliders do the job with trees just fine.
On the second issue: Yes. This is very common for trees or plant textures. This is due to their material settings. (Transparent > Cutout > Soft Edge Unlit, etc.) You can go into their settings and play around with it to change that to actually go about with the lighting/shadow. For regular materials, it’s usually Standard by default. But standard isn’t usually what you’d want to use for trees if the trees’ leaves aren’t made up of actual polygons. You’d want to go to Legacy Shaders > Transparent and play with those settings so they will be lit, have specular, etc.
Speed Tree is usually a good way to go for backgrounds or far distance, unreachable areas in your game without colliders. (For example in Slender, trees on the other side of the gate) This is because of their performance. They are usually very good for performance if you use the correct polygon count, etc. Which shouldn’t be a lot at all. But for trees that are closer, you might want to actually use more detailed trees. And hand-placed on your scene. Of course speed trees can still do the job. Along with Speed Grass but it’s all on your liking.
Tree Creator you really only need to know basic modeling practice. Since you aren’t actually creating the trees yourself, instead you have a preset polygon and it’s shaped, in Unity’s Tree Creator, to look as if it’s a tree. And, yeah, it’s doable even for people that aren’t very good with 3D modeling skills. But few basic knowledge is still recommended when using speed tree.
Hope this little guide helped.
EDIT: Yeah. The material on the trees reacting to your scene lighting (Not really your character flashlight much, just depending on how many trees the flashlight is hitting) can slow the game if it’s heavy shadows everywhere with hundreds of trees that has reacting material. If you absolutely need shadows, or darkened area such as in Slender - It’s recommended you go into Windows > Lighting and changing the Ambient Intensity to 0. This’ll create completely dark areas where no light is available.
Seems like quite the incorrect blanket statement…