I'm using PayPal webhooks to get subscription information automatically.
However, we have to wait about 20 seconds between the payment and the subscription activation.
Is it because of the sandbox environment? Is the production environment faster?
This is important because the customers have to wait and if waiting time could be avoided, it would be better.
The sandbox is slower in general, but you will need to test yourself in live -- and the speed of asynchronous notifications vary in different conditions.
If you need a faster notification, what you can do is have the client-side onApprove event call your server (with a JS fetch similar to this demo, plus a body payload if desired), and have the server route that handles that fetch use the Subscriptions API to get the status of the subscription, and see whether it is in fact active in that API response direct from PayPal.
Such a client-side trigger of a server route would happen in parallel to waiting for the webhook notification, so whichever completes first will mark the subscription as active in your records. This way you are not relying on either the client-side trigger nor waiting for the webhook, but rather whichever happens first.