It's hard to say exactly how much it'll affect execution speed, but your code isn't written how I'd write it.

First, I'd try to minimize the complexity of the initial condition. I'd only return if root was a null pointer. Unless the tree is tiny, you're probably going to gain more from a minimal test than from being able to skip a level of recursion.

Second, when you need to swap things, it's better to use swap to do the job. One of the interesting points of swap is that it's fairly common to specialize it, so you don't want to call std::swap directly. Rather, typically want a using declaration to bring std::swap into scope, followed by a call to swap with an unqualified name, so it'll find a version of swap in the target namespace (if there is one), and fall back to std::swap (if there's not).

using std::swap;
swap(root->left, root->right);
invertTree(left);
invertTree(right);

This may or may not improve speed for your particular case, but it certainly can in some cases (often enough that you usually want to follow this basic pattern for swapping items in C++).

It's hard to say exactly how much it'll affect execution speed, but your code isn't written how I'd write it.

First, I'd try to minimize the complexity of the initial condition. I'd only return if root was a null pointer. Unless the tree is tiny, you're probably going to gain more from a minimal test than from being able to skip a level of recursion.

Second, when you need to swap things, it's better to use swap to do the job. One of the interesting points of swap is that it's fairly common to specialize it, so you don't want to call std::swap directly. Rather, you typically want a using declaration to bring std::swap into scope, followed by a call to swap with an unqualified name, so if the namespace of the objects being swapped contains a swap, that'll be found via ADL, and otherwise lookup will fall back to std::swap.

using std::swap;
swap(root->left, root->right);
invertTree(left);
invertTree(right);

This may or may not improve speed for your particular case, but it certainly can in some cases (often enough that you usually want to follow this basic pattern for swapping items in C++).