Backup-SPSite - will backup your site collection and not sub site.
To Export a subsite you need to use
Export-SPWeb, you can use below optional parameters to improve performance.
CompressionSize Optional System.Int32
Sets the maximum file size for the compressed export files. If the total size of the exported package is greater than this size, the exported package will be split into multiple files.
Either enables or disables file compression in the export package. The export package is stored in the folder specified by the Path parameter or Identity parameter. We recommend that you use this parameter for performance reasons. If compression is enabled, the export process can increase by approximately 30 percent.
Disable log file with parameter
Suppresses the generation of an export log file. If this parameter is not specified, the
Export-SPWeb cmdlet will generate an export log file in the same location as the export package. The log file uses Unified Logging Service (ULS).
It is recommended to use this parameter. However, for performance reasons, you might not want to generate a log file.
Reference - https://technet.microsoft.com/en-us/library/ff607895.aspx