Amazon has a limit on all data retrieved from its servers. The limit is rather high, so users of Seller Metrix never notice this except during the onboarding stage.
As a rough estimate, Amazon allows Seller Metrix to process approximately 1,000 orders per hour for each Seller Central account.
So as long as you have below 1,000 orders per hour (16 orders per minute) you will never experience any delay on Seller Metrix.
However, when you sign up to Seller Metrix for the first time, the system will import your orders in the past 60 days which may take some time. Take a look at the image below to see how long time your onboarding will take approximately.
In some rare cases, the onboarding can be delayed further because of various reasons, but as a rough guideline take a look at the image below for processing 2 months of data during the onboarding phase.
Full onboarding
When you have added a payment option, the system will retrieve an additional 16 months of sales data. To estimate the time this will take, you can take the above numbers and multiply by 8.
So if you have 1,000 orders per month, the full onboarding will take around 16 hours to complete.
To summarize, the onboarding time depends on the size of your sales history. It can take several hours to fully import your historical data if you have a lot of previous orders. But if you are a brand new seller, it will only take a few minutes.