In version 14, after entering item code in first column the item name and other details was fetching instantly and can be quickly move forward quickly by clicking on tab button, now in 15 after entering item code in first column then clicking tab button instantly, its getting selected some other item. so we need to wait till the suggestion box to come in the item code column, also the price editing performance gone down in sales invoice.
Check the Server CPU usage and background processes. Remove the irrelevant processes
This issue got in frappe cloud.
There are no irrelevant processes there
You can debug using the monitoring tool of Frappe.
https://frappeframework.com/docs/user/en/profiling