I had exactly the same problems with IE8 tabs being very slow to close and occasional IE8 crashes/lockups on my Windows 7 Ultimate x64 system. My system is fully upto date with Windows Updates.
Also, as mentioned by others, disabling the lastpass IE Toolbar resolves the problem, so it is definately a lastpass toolbar related issue.
I have now installed the 'x64pre' version mentioned and the IE8 tabs problem has gone. It is too soon to see if the crashes/lockups have also been fixed. If not, i will report back.
So far, so good. Thanks.