I understand that creating objects within UE4 with blueprints will run roughly 10 slower than creating them via C++, but what happens when you mix the two? Will I experience a noticeable speed reduction if I create my objects/actors with C++ but allow some aspects such as run speed to be updated via a blueprint?
Only the actual updating of run speed will be slower, and only when you do it via blueprints. The object will not be generally slowed down by having it acessable from blueprints. So it is totally fine to do a lot of stuff with blueprints and only use C++ when the performance matters.
Also: You need to get pretty deep into blueprints (running advanced AI and physics behaviors) before the “10x” slow-down actually becomes noticeable on a profile.
Don’t solve a problem that doesn’t yet need solving 
One thing to beware of is doing too much on Tick. But if that causes problems the first step is to remove everything that don’t really need to happen each tick.
Ok cool. I’m working on writing my own car game and i was debating if I should use blueprints to adjust things like acceleration curves and such.