Have you opened a Support Case? If not opened, for more information, please contact the SD Technical Support team for best assistance and troubleshooting:
The slow loading, deleting, and random long pauses in playback on a 4TB Extreme Portable SSD may be due to hardware limitations, storage capacity fragmentation, software issues, background tasks, file system issues, or overheating. To resolve the problem, consider updating firmware and drivers, performing defragmentation, checking for background tasks, enabling the TRIM command, using a compatible USB port, and ensuring sufficient power supply.
If you have any data lost issue visit SanDisk Extreme SSD Failure: Tips to Prevent from Further Damage