The function is a scalar UDF which is a performance problem in and of itself. That, notwithstanding, things don't just suddenly start running slower unless something actually did change. Perhaps the target table of your load finally reached a tipping point where the existing statistics are no longer valid causing a change for the worse in the execution plan. Perhaps the server is using more memory for other things than it used to.

I recommend that you first rework the scalar UDF and the code or process that uses it to be an Inline Table Valued function that returns a scalar value. That will take care of some of the performance problem. Once that is done, I'd re-evaluate any slownness that remains.