The computer will be slow overall, not just on the internet or inside of any specific software.
Run CCleaner and TFC (Temp File Cleaner). Is there plenty of space available? Open “My Computer” and check.
Run a chkdsk on your OS drive. Many times you may have a HDD that is starting to fail.
Most of the time this is a common sign of a virus, I recommend you scan with MalwareBytes AntiRootkit and then MalwareBytes Regular (MBAR and MBAM).