I just got a new modem for my higher internet speeds (bumped my service to 30mbps down / 5 mbps up), but I needed to retain my phone line, which wasn't an option for my new modem. So I purchased a coax cable splitter and two short coax cables from Monoprice, and used both modems; the old for phone, the new for internet.
My biggest concern was whether my speeds would be affected by the degraded signal or not. Multiple Google searches, looking for things like "will a coax splitter lower my internet speeds" yielded mixed and confusing results at best, and no hard numbers on what to expect.
So I did a little testing of my own, before and after installing my new modem / coax splitter. I ran each test about 10 times and averaged out the results.
(PING / DOWN / UP)
Old modem, no coax splitter:
24ms / 25.63 mbps / 1.03 mbps
New modem, no coax splitter:
23ms / 30.14 mbps / 5.03 mbps
It appears my tests with the old modem were flawed. The download speed was limited by the modem, but the upload speeds appeared limited by me not doing a power cycle after the 5 up was added to my service. Nonetheless, this shows how a proper modem can mean the difference between fully using your service or not.
A helpful side note, always run a power cycle for your modem and then any routers when you change your internet services. I say modem first and then the router because I did them both at once and it caused my router to have trouble resetting correctly. Back on topic...
As for the two-way coax splitter, the results were virtually identical. I actually got a slightly better result from the test on the coax splitter, just by chance. I'm sure results will vary, based on the signal strength provided by your ISP, but chances are you will not know the difference.
The splitter cost me $2.01 and is rated for 2400MHz.