FYI, SSDs don't spin as there is no mechanical movement within them. How are you measuring the speed of the external SSD?
What is the exact make & model of the external SSD (provide a link if possible to the product page)?
How is it connected to the Mac?
What is the exact model of the Mac and version of macOS? You can get this information by clicking the Apple menu and selecting "About This Mac".
What file system is being used on the external SSD?
Make sure to disconnect all other external devices in case one of them is causing a problem. Connect the external SSD directly to the Mac. If you must use an adapter, then maybe try another adapter. Try using another USB port, if on a laptop, then on the other side of the laptop.
If you run the third party app EtreCheck it will provide the answers to some of these questions as well as allow us to see whether any third party installed apps may be interfering with the normal operation of macOS and also provide some information about the performance of your Mac which may be related to the SSD speed issues.