I agree it has slowed down noticeably in the last couple of weeks. If you look on process monitor its opening and closing 2 million registry keys that contain blacklisted domains (in HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap\Domains). If you delete these the slowdown stops, but you might not be as protected against spam\phishing\whatever. In any case it would probably be smart to make a backup of that key before deleting all 2 million entries in it.
