# Why does changing the evaluationDate multiple times lead to a performance lag?

I am simulating an swaption strategy through time. Following the examples in the Python Quantlib cookbook, as I progress through time I am updating the internal evaluation date

ql.Settings.instance().evaluationDate= todays_date


The first few iterations its reasonably fast, but after about 10 or so this line of code takes forever. What's going on? How can I keep it fast?