Safari slow

Hopefully one of you smart folks can help, In the last couple of weeks Safari on my iMac has been slowing down, clik on something and nothing happens, several second delay. I checked the activity monitor and found the webkit activity was at 127.0....????? Turned off the webkit action and computer resumed normal speed. In a short while though the "slow" returns and webkit activity is once again "big". Ideas ?? Fixes ??