Hi there and welcome
iPhones do not get viruses but the firmware can crash.
Can you hold the home and power buttons on the iPhone until you see the Apple Logo, then release.
Let the iPhone restart and when it has, test and tell me if the speed has not increased.
Let me know the results please
can you go to Settings, General, Reset, Reset Network Settings.
Let this complete, repeat the initial instructions I did and tell me if it now improves please.
Where is it running extremely slow? In a particular app?
We will then need to reset the phone
First, have you backed up the phone to your computer via iTunes?
Do you have data you need to keep on it? If so, what data?
Correct, any data on the phone. eg; Contacts, photos ect
ok, go to Settings, iCloud and check if its all on on the phone