That sounds like either:
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The drive is spinning down and going to sleep, and you have to wait for it to spin up again the next time it is accessed.
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Antivirus is getting in the way somehow and blocking things for a long time that would normally be fast.
If neither of those explain things, please link your account and send some Process Monitor logs and we might be able to work out what's happening on the system that is slowing things down.
Checking the same thing in other software in case it's not limited to Opus is also a good thing to verify, although it might be difficult if it never happens twice in a row.