Your Ethernet cable 'could' be the problem but note that unless it's physically damaged (kinked up or something like that) the max length is 328 feet/100 meters before you need to start worrying about any degradation, and as far as type the general specs are Cat5 will go up to 100Mbps, Cat5e 1000Mbps, Cat6 10Gbps so if you're using a Cat5 cable might be time to just get a newer one.
Also, I wouldn't opt to use WiFi over wired. With an Ethernet cable it's always using a full duplex connection, send and receive signals take place at the same time. With WiFi, it's always half-duplex, signals can only be sent OR received. All things being equal (measured on the same LAN), wired will always be faster than wireless. When content is being streamed to your M8 it's almost all one-way anyway so the duplex vs half-duplex issue isn't a real issue, but WiFi also has inherent latency and noise floor issues that wired does not.
As for having more than multiple devices using your WiFi at the same time, it's probably more a matter of bandwidth than number of devices. On even a mid-range wireless router you can get over 30 active connections without a problem, as long as there's the enough bandwidth. Some cheap models are limited to 10 or less. With a 145 Mbps connection you should be OK having people watching different things, unless you're all into high definition content. For instance a standard def Netflix show only requires 3 Mbps.
Regarding your WiFi router, do you have your own or is it an ISP-supplied modem/router? What model I.D. is it?