Dear Soriano Gamale,
Thank you for posting in Microsoft Community. We are happy to help you.
Based on the description, when pasting a contents from Excel local copy into SharePoint Online Excel file xls, it keep freezing or not responding. I just want to confirm your Excel file format is xls saved in SharePoint Online library and you open it on Excel Online?
I did the testing on my side, I created Excel xls file and upload it to SharePoint library, I open it on Excel Online and I received a message "Edit this workbook" upon clicking the button Edit, I received another message File conversion.
In order for us to edit xls file format, we need to convert it to new version which is xlsx. This is online version and it will allow us to edit the file using Excel Online once converted to xlsx.
After the file being converted to xlsx, I can paste a contents from my local Excel xls to the converted file xlsx using Excel Online.
I would suggest you to convert the file to the latest version in order to edit it using Excel Online application or open it on Desktop.
Advantage of xlsx, is can do re-time collaboration and auto save will be on.
I would suggest you also to use modern and updated browser like Edge Chromium or Google Chrome.
We look forward to your response. Thanks for your cooperation and kind understanding. Stay safe and healthy always.
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Sincerely,
Zee | Microsoft Community Moderator