You can remove the check mark on Clear history on exit.
Your PC may be loaded with trojans, coz of those porn sites. That may cause system slow.
Update you antivirus & Run a complete scan, if you don't have any antivirus/malware,
download & install, run a scan
1. Avira Anvivir - http://www.free-av.com/en/download/index.html
Avira is best & light solution for excellent overall system protection & internet security with real time updates, .
2. Malwarebytes - http://www.malwarebytes.org/
Malwarebytes is one of the best & effective antimalware tool out there.