This happens due to the set of the filesystem features are not enabled during the mount time. The below features commonly influence the extX filesystem performance:
This feature disables registering a file access time.
This feature allows metadata to be written asynchronously.
This feature enables/disables the use of write barriers in the journalling block device code. When turned off, it allows you to bypass strict order while writing journal commits.
The last feature is enabled by default in ext4 and might be the reason for the visible performance degradation compared to the same device formatted with ext3.