I'm a pc newb but is this decent for gaming or will I be experiencing a lot of lag?
Side Q: Whenever I upload something it tends to upload at 1 mb/s and my downloads are almost always 2 or 3mb/s, so what does that 21mb/s really mean?
Your internet is more than adequate, unless you are playing with someone on the other side of the world.
As for your second question, MegaBIT (Mb) and MegaBYTE (MB) are different. The capitalisation is important. The speed from speedtest and your ISP are measured in Mbps, while most downloads will probably be measured in MB/s. On a good day from a good source, you should get about 2.5MB/s
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