What is slow and how slow is it? Typically, this is the invoices manage table screen. So how many records in the database table?
SI is using the DataTables. It uses ajax to supply the data. In my testing, it is not the server that is the issue. It is the client rendering the data. Also, the feature is set to present the table when the first page is formatted. That is, it doesn’t have to wait for all the data to be transferred and rendered before displaying the first page.
Let me know what you have and we’ll go from there.