The Air Explorer seems cannot be tuned, and their default options for s3 are not optimal for Storj DCS, thus it cannot be used for big files transfers.
You can still use CLI tools like rclone.
Since you several times mentioned “slow”, I can assume that your upstream bandwidth is not great, in this case I would definitely recommend to avoid a native integration (because it opens 110 connections to nodes for each segment) and use Gateway MT instead (it opens one connection per part), thus you need to configure rclone with a Gateway MT too.
See also
If you want to use UI, you can try to use FileZilla PRO, it has s3 support or MSP360, or any other S3-compatible tool which has more configuration options, than Air Explorer.
Or just try to use an Objects browser.