It seems you can work around the slowdown by disabling:The only solution is to disable the Immunization (portions of it) for Microsoft Internet Explorer 8. Microsoft is persistent about changing their code (apparently, IE8 'slows-down' when there is too many entries in it's restriction zones) for Spybot.
I don't see the use of Immunization when IE8 has the SmartScreen Filter, which filters and blocks out malicious sites (including .exes). SmartScreen should be more than adequate to take on Spybot's role.