Weak security issues

Well this is a question that I hopefully hope some of you could have encountered and maybe relate! So basically my internet had been working perfectly fine and it was very fast until a few months ago it meet giving me this “ weak security “ alert under the internet I am currently using, I ignored it at first but then I slowly realized how slow my phone and my device was getting. Every time I played games or watched YouTube it would be somehow slow and so I worried! Has anyone encountered this problem and what was the simplest way to fix it