My iPhone 7+ is becoming slower.

About a month ago I bought an iPhone 7+ 128gb and recently I've noticed that it has been glitching and also it has been slowing down. I don't know what caused this since I take good care of my phone. I thought that memory caused the problem but I only have about 21gb used as of this moment and I frequently update my phone. How can I fix this problem? Can certain apps cause this problem? Do I have a virus and if so how do I remove it? I just what to mention that before this phone I had an iPhone 6s and I transferred the whole phone to this one. Thanks for answering my question, hope to hear from you soon.