Try to use Auslogics Disk Defrag Free as a defragger.
It helps speed up your system.
For browser extensions,
I would suggest Web of Trust (WOT), it helps by blocking bad sites based on the reputation of the online community.
For your DNS,
please install 'DNS Jumper', and test for the fastest DNS. Mine is Google DNS.
This improves your browsing speed a bit.
You can also choose Norton DNS or Comodo DNS, they have anti-malware properties and will automatically block harmful websites.