It is very common and you have nothing to worry about. Your router is dropping certain traffic. Its known as TCP Reset (RST) packets, it flags them as a attack and then drops them. Its very unlikely that your being targeted to DOS attacks. So you slow internet has nothing to do with that log. If it was a DDOS attack, you would have no internet whats so ever while you are being attacked. That's why they call DDoS (Denial of Service) Network printers, TV, Laptop, Computer tablets, basically any device in your home can cause these logs.
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