Ok, I have contacted the Google Ads Product support team and they were not really helpful.
They told me that this is what it is, and that's it.
Now, this is a HUGE disadvantage, having to wait 6 hours to upload conversions. As mentioned, other marketing platforms such as Facebook or TikTok allow developers to upload conversions in real time.
Why this is a HUGE disadvantage? Because there will be always a 6 hours delay in the data we are seeing in our Google Ads dashboard, and that will make decision taking harder / unreliable for us (when to increase budgets, when to add a keyword to the negative list, when to change an ad etc.).
By using the Google Ads API we thought that the way to handle conversions was going to be similar than using the Google Ads Conversion Pixel, but apparently, it is not.
By having to wait 6 hours to upload conversions, it makes developers not want to use the API, because the data collected via the Pixel is real time = more reliable.
And this is a pure technical thing I guess, because if Facebook or TikTok are able to process their click IDs in real time, why Google can't?