@ocnoglittle Hellooo ^^
1. At this moment, they are both just about the same. There might be situations where one is slightly faster, but since there is a very little difference, I would just go with the one that's more suited for your game ^^
2. The only reason not to weld something, is when you want to interact with it somehow, or you want for single blocks to be breakable. ^^ As far as the trees go... mm. I don't think it will make much of a difference since there will remain more or less the same number of colliders (unless you turn off the colliders, of course, :D). But I would say that you can go ahead and weld them together, you can never go wrong with welding, just remember that they will act and receive damage as one object from that point, so probably make them indestructible.
3. Will try to incorporate them into one of the following videos ^^
4. I'm not sure! I didn't really come up with anything so far, but I will make sure to mention it if something comes up ^^
5. Performance monitor is really just for monitoring some unexpected spikes where you unknowingly spam 1000 entities at once etc. Generally, you'd want frame time less than 300ms, and polygon count is more or less just for informational purposes now.. Script profiler is good for checking how many times have the functions been called and how fast your functions are. I haven't been using the perf monitor that much to be honest, I really use it just to check for some big spikes that would show some greater issue in my script.