My libraries are on external drives, never had a problem with them - they used to be on USB 2 ports they're now on USB 3 ports.
When I was adding large quantities I found I got more done by setting Preferences->Miscellaneous->Max. simultaneous conversion/news download/plugin jobs: to 1 and not checking Limit the max. simultaneous jobs to the available CPU cores
And don't do any automatic conversions when you add - that setting is in Add Books->Adding actions.
BR
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