70+ MBPS at the cable modem gets shared by all the devices connected to your router (which might not even have that much throughput with only a single device). The router itself might have a much lower maximum rate than the cable modem or the service rating.
What the router can handle gets split up among everything connected, determined by maximum speed of each device and how busy each is, and how much they are throttled by WiFi bandwidth on however many radio channels.
Add up the bandwidth being used by each device, and you'll have an idea of how much of that 70 MBPS (or lower router limit) is being used where. Expect as much as a 50% cut in the total if the router is really busy, and most devices are on WiFi, because of the wait times built into collision detection and prevention protocols.
When collisions are high, the whole system can come to almost a standstill, everything connected spending more time in "wait" than sending or receiving packets. When be had family gatherings at my brother's house, the single-channel 2.4 MHz WiFi would come to a standstill about when the smartphone count reached 10-12 devices busy browsing or facebooking.