Can I throttle SLIM Companion Explorer to avoid placing too much load on the SharePoint / OneDrive for Business system?

Yes, the tool provides various options to throttle the load such as number of parallel requests, wait time between successive requests, limited number of retries for failed requests, maximum file sizes, maximum upload volumes, …

The number of threads can be configured for all users via the properties.txt file.
Users can change part of the parameters in the "Settings" section if they are not greyed out. Changes are only valid throughout the current session. Closing the browser will reset the values to the original default values.