Throttling is completely normal, so if you experience it you shouldn't worry. Throttling is an essential security mechanism that ensures the service offered by your server remains healthy and responsive. It is referred to as "throttling" because it is limiting the work load coming through the server. Without this, the server could become unresponsive or even crash completely - causing service interruptions for all users.
Throttling when running an Office 365 migration
When running your Office 365 migration you may get an error indicating the status code 429 Too Many Requests.
This error does not indicate that your migration has been stopped, it serves to notify you that your tenant is under a lot of pressure and therefore it cannot handle the amount of requests required at the moment. You need to wait, as Sharegate will retry the procedure until the server is ready to handle the amount of requests required to proceed with the migration. You can cancel the procedure at any time if you prefer to attempt the migration at another time.
The cause and effect of throttling
Sharegate's goal is to complete your migration simply and efficiently - and that means processing requests as fast as possible.
Due to this, there might be a heavy load on your tenant that contributes to throttling. However, the migration procedure alone is not usually the sole cause of the throttling. Often, the amount of users on your tenant during business hours generates significant load as well. Other causes, such as backup applications running simultaneously to Sharegate contribute to the strain as well.
Reducing the load to prevent throttling
The best way to prevent throttling is to schedule large migration procedures outside of business hours (overnight or weekends are ideal).
If you are dealing with a business that runs across different time zones (thus eliminating the opportunity to run the migration with low server load), review the applications that use your tenant to run. Limiting background application activity during the migration procedure reduces the chance of throttling.
If neither of those options are possible, it may be best to split a large migration into smaller intervals. Migrate a bit at a time, and the request load on the server will be significantly less.
You can check out Microsoft's SharePoint Online and OneDrive Migration Speed article for information on the factors that affect migration speed and some best practices.
Throttling may cause frustration, but at the end of the day it means that your Office 365 tenant is being used to its full potential.
Experiencing some throttling is normal, but significant throttling indicates an issue. Significant throttling consists of more than 30% of request being throttled for a period of 30 minutes or more.