As already answered, it's because you have a mismatch in the speed between the far device and your computer.
- Just try all the speeds in (my suggested) order: 9600, 1200, 115200, then all the speeds your terminal will allow.
- As a heuristic, if you get too many characters, try a lower speed; too few, try higher.
- Use handshaking "none" unless forced otherwise (thanks commenter Andrey).
- Keep it at 8N1. (If you get the right number of characters but some are wrong and some are right, then you have the right speed but wrong parity, extremely unusual in 2018. Try 7e1, 7o1.)
Just to add that serial ports are for specific purposes, and don't get frustrated if you find it slow, because debugging is always slower than anything else.
It is for this exact reason that many organisations fix a lowest-common denominator speed for serial ports -- often 9600 8N1 -- so that you never spend any time debugging this. The reasoning being if you're using the serial console, you're doing something which is rare and can't be done over the main network.
One organisation I worked with had it on their specifications list: "equipment must be delivered with default serial console speed at 9600/8N1", which could get negotiated down to "equipment must be able to be permanently configured to 9600/8N1".
A number of otherwise-excellent manufacturers lost contracts because of this. I know of one manufacturer which would have won the edge switch contract, but its equipment had fixed 115,200 baud which couldn't be changed -- I wrote to them complaining (2016), I hope they've fixed it now, but they lost all those sales.