what causes my mobile phone to slowdown?

Restart your phone again. Maybe It can be helpful for you.

Uninstall all the bloatware. Check what is running in the background.

What's your phone model?
Huawei comes with bloatware and you can delete few unwanted app.

It was helpful.

I think some application is running in backend and do not close them thats why your mobile phone gets slow

Sometimes too much data stored in storage space can cause such thing but as you have mentioned that you have bought a new phone, then you can read this article to deal with speed issues: https://www.androidcentral.com/ten-quick-tips-make-your-android-phone-faster

I think there is some malware that is causing slow down of you cell phone performance don't try any malware cleaner because they will make your phone slower try to restore your phone through this you will be lost your data but hopefully your phone will be optimized.
if you want to restore your data then please check this audio video and all files recovery thankyou

do hard reset it helped me a lot