Sounds like you need a #12 OnyX cache cleaning, however if the machine is slow on all things it could be indicative of failing hard drive, if overheating also, a indicative of a dead fan or sensor and your getting CPU throttling.
You won't know more unless you backup and do some investigation
Do the emergency backup in addtition to any other backups you have
Then run through this list of fixes
You might have a network or wifi security issue if it's only occuring with Internet programs and also on other devices (failing router or modem)
And if you want to provide us some information about your machine so we can assist further
If it's all too much for you, there are local PC/Mac techs that can do data backup/recovery and suggest a fix, reinstall your operating system etc.
If your machine is still under warranty/AppleCare, then take it to Apple, but they don't do data recovery.
ds store, greeting; It is good to see that you have abandoned that prior horrid avatar. I do note that is bears a strong resemblance to mine. I will not raise any objections at this time, but do understand I have an unimpeachable reputation to maintain. Should any confusion occur due to misdeeds by you, I will refer the matter to my attorneys.