The execution will first trigger a package validation process to ensure the package is OK. For large packages, especially with a large amount of files, this process can take time.
In version 8.5 RU2 the package validation will be skipped if the package download was completed within 15 minutes of the execution. Package validation is already part of the package download.
This issue may still occur if a user defers execution past the 15 minute period. If they do, we must ensure the package is valid before executing to avoid security issues or corruption.