Change checkout fields text with hooks

Hello Mike,
I tried the Say What? plugin.
It works for me in this case, but I can easily think of places where I’ll translate “too much”…, the “context” is a good idea, but it is barely used or not used at all in most cases (or am I wrong?), so basically you can translate according to the domain, which in woocommerce case means if I’m translating the word “Product” for instance, I’m translating it all over the store and not just in the checkout page.
One more thing I’m concerned about is speed. I think this kind of on-the-fly translations will slow down the website as they are happening in runtime. I’m guessing hardcoding the texts will be the fastest, but Won’t hooks be a better way than on-the-fly translations?
Anyway, will you be so kind and help me getting started with an example for using hooks in this case?
For instance, changing the “account password” label, or the “Billing Info” heading, both in the checkout page…
Thank you!!