"We've built 1.1.1.1 to be the Internet's fastest DNS directory. Don't take our word for it. The independent DNS monitor DNSPerf ranks 1.1.1.1 the fastest DNS service in the world."
I had this problem several times. I was lucky to find a good list of the possible reasons here https://www.firesticktricks.com/why-is-my-internet-so-slow.html. Usually, I just check it out and try one by one until I find the solution. Hopefully, it will be useful for you too.
Choose a variety of DDoS Protection and Mitigation Service providers at Fastblue to keep your website, webmails and business critical applications running all the time with preventing from DDoS attacks. The overload of communication requests from hackers is what slows your website or email exchange. Imagine the damage that could be done to your company and your productivity with no website or email for hours or even days. The best way to fight against DDoS attack is to prevent it with Fastblue's variety of DDoS Protection and Mitigation Service providers.