Will it slow down my Wi-Fi?

No. The ticker asks its server for one coin’s price and chart as each coin’s turn comes round, never more than a few times a minute, and each answer is a small fraction of what a single web page costs. It holds itself to a budget of fetches per minute even when you step through coins by hand. The only sizeable transfers are a firmware update, about a megabyte, a few times a year, and the coin-logo pack, under two megabytes, when it changes. Nothing streams. See How often prices refresh.