AMSI scanning uses the Actions and Exclusions specified for Standard process types in the Process Settings section of the On-Access Scan settings. Therefore if something is excluded here, AMSI should not be scanning it either. If you are seeing that, it would be a problem and would need to be investigated.
I have an SR open for this. The directory in question has indeed been added to the 'Standard' section for the OAS to exclude yet AMSI is still intercepting things, thus severely slowing down the scripts.
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Something else I just thought of.... maybe we'll need to add some more directory exclusions?! Maybe the one that was added just wasn't enough? Our scripting people said that the dir they gave us was the one they use so who knows.
Thanks for sharing the SR. I've taken a look and can see you mentioned that these are fileless files still being scanned. In which case - as you seem to have already found yourself - the exclusions would not be valid.
AMSI excludes most files that are excluded as per OAS policy however some scripts i.e. fileless ones are not excluded from AMSI. You could submit a PER for the same however I can confirm this is as designed to ensure your systems remain protected. I do however understand the problem you are facing will be quite difficult to overcome - it may be something you need to look at with your team and change the way they achieve the intended goal.