You will be accessing a live API from NCBI. Note that the performance of the system may vary during the day. Furthermore, you are expected to be a "good citizen" while using it. You will find some recommendations at http://www.ncbi.nlm.nih.gov/books/NBK25497/#chapter2.Usage_Guidelines_and_Requiremen. Notably, you are required to specify an email address with your query. You should try to avoid large number of requests (100 or more) during peak times (between 9.00 a.m. and 5.00 p.m. American Eastern Time on weekdays), and do not post more than three queries per second (Biopython will take care of this for you). It's not only good citizenship, but you risk getting blocked if you over use NCBI's servers (a good reason to give a real email address, because NCBI may try to contact you).