On average, generating a Image will take around 1-2 seconds. The time can vary significantly depending on what kind of call you're performing.

HTML with no external dependencies is the fastest
Using an HTML call that does not have any external dependencies such as images, CSS, or other assets that get fetched from the internet. Depending on how much content gets generated, the Image will get generated extremely quickly and typically around 1 second.

Using an external URL is the slowest
Using an external URL to generate an Image could be the slowest possible option. It is not always the case because it depends on how fast the external server is, how big the payload is, and other various factors. But on average, using an external URL will be slower than using embedded HTML.
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