You can find more info on the number of supported rows and columns in Restrict the Number of Visible Rows and Columns
help article. Note that this would not change the actual number of the imported (and respectively added to the document) rows and columns.
RadSpreadsheet is generally capable of importing large spreadsheets, and I am not sure what exactly is causing the OutOfMemoryException in this particular case. I would suggest you to open separate support ticket and attach the problematic document, so that we can investigate it further.
About pasting performance, we will also need more specific steps to reproduce the issue:
- What program are you copying from?
- What is the document you are copying from (it should be best if you attach it)?
- What is the exact copied content?
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