Due to internal processes including recalculation of the parent Order values, deleting child records like an Order Product can take a while.
When you delete a single Order Product, the system will recalculate the parent Order's total, etc. This happens for each record, and deleting multiple records of course takes longer.
There may also be other processes happening - either custom or system processes. You can check if there are any custom ones, but the system processes are largely a black box.
I have seen situations where a client occasionally needed to create an invoice with over 10,000 lines. Since creating each line triggers a recalculation, normal automation options were timing out. I wound up creating a console app to add the lines to the monster invoices in batches.
