Do you run the add on ‘NoScript’, AdBlock and there are some others that as add on’s cause problems with some peoples machines and not others. Personally , I have been having this problem lately, “NoScript” was my problem.
Disable all your addons, if this fixes your molasses problem. Then turn on each add on one at a time and figure out your culprit.
In addition viruses, malware can also cause this.
I watch some online TV and NoScript was messing with it.