If I create a tool which makes it easier to complete my work, and complete it much faster than before, the client does not need to know about it. (Provided you aren't under a work-for-hire agreement).
How I do my job, the speed at which I do my job, the tools I use to complete my job, what hours I work and where those hours fall, are all my choice and the core of what freelancing often is. If none of these are true.... are you certain you aren't under a work-for-hire-agreement? (In which case, the client may own this new "tool".)
I'm 200% faster at completing tasks today than I was 5 years ago... but I should never make less money just because I can complete something at a faster pace. Never penalize yourself for being good at your job.
Where this gets tricky is if you are under an hourly payment agreement. If work took you 4 hours yesterday, but this new tool means it'll only take 30 minutes today, you're in a pickle - at least ethically.
You could keep your rates the same and just invoice for the 4 hours it would have taken you to complete without your new tool. Its ethically questionable, however, again, how you complete your work is your business. Developing something that the means you don't need 4 hours doesn't necessarily mean the client shouldn't be paying what 4 hours of time is worth.
You could raise your rates considerably and cut the reported hours down to 30 minutes - this may be problematic. If you start slashing your expected or anticipated work time, you may find that when there's a problem you don't have adequate time to deal with things and clients are harping at you because you've been so fast in the past.
Ideally, the balance would be somewhere in the middle. You raise rates to appropriately compensate you for your speed.. and you cut the necessary turnaround time down to allow the client to benefit from the new speed increase as well. Both you and the client get something this way. Client gets faster turnaround, you get more free time without any loss of income.
I tend to not work hourly in most cases due to this. I work on value-based pricing. I've quoted a price to my client. The client is happy with that price and will pay that price. How I complete my tasks is entirely my choice - whether it takes me 16 hours or just 1, the client doesn't care they will still pay the quoted price. If I come up with a huge time saver, the client still pays what they've agreed to pay... I merely may be working far less to deliver what is expected. Again, I refuse to penalize myself for being good at my job. It's my opinion that for most freelancers working for hourly pay is never a great position to be in. Fire your hourly rate if possible. It's holding you back.
I don't think this question could really definitively be answered without fully understanding any contract or agreement you have with your client.