if it's serious and what to about it, depends on the exact error and warning. You didn't tell yet.

General slowness of a PC generally is caused by:
- not enough RAM
- too many or too heavy background processes
- malware
- slow CPU (on a netbook for example)

Slowness of a browser generally is caused by:
- add-ons
- malware
- slow connection with Internet
Trying another browser (Firefox, Chrome) helps to see if it's IE-specific.

So please tell much more.