Answer for Router supporting 100 mbps spped
The DIR-816 only has a 100 Mbps Ethernet port for the Internet connection. Therefore it will never give you the full bandwidth of your 100 Mbps connection since 100 is a theoretical maximum. I would suggest a router that has gigabit Ethernet ports as they should have the throughput to handle the 100 Mpbs connection. I generally recommend something like the Linksys EA3500 as a starting point. That being said, if your speed test results are based on wireless connections, then it is possible there is some wireless interference that is causing the slow results. You may want to try testing your iMac through an Ethernet cable to see if you are getting the same results. It is also possible that your Internet provider is not providing you with the full bandwidth you should be getting.
Read this article for more information on troubleshooting your Internet connection: https://marcelbrown.com/2015/08/13/is-your-internet-speed-not-what-it-should-be-how-to-test-and-isolate-the-problem/