BEFORE you put the splitter on, visit this web page:
http://192.168.100.1
its the web page of your cable modem, and visit the 'signal level' tab.
You can easily Google up the good and bad levels, won't waste space on that here.
Then AFTER you insert the splitter, making sure all taps are terminated (plugged into something), check that web page for changes and verify the levels are good according to your Google search.
... Happy Splitting ...