i think my isp is faking the speedtest results

it doesn't feel like 20Mb/s AT ALL
it feels more like 4Mb/s
that's because it isn't.
speedtest is in megabits per second. not megabytes (which is what wer would be familiar with)
21 megabits is about 2.6 megabytes.
keep in mind google fiber is non a gigabyte a second. it's 125 megabytes a second.