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When using a table with a reasonable amount of data - 100 rows by 50 columns - I notice that IE8 performance degrades unacceptably (only in IE8 standards rendering mode). The CPU usage spikes to 100% and the browser becomes very sluggish. Increasing the amount of data in the table amplifies the sluggishness. This became apparent when applying a background color on hover on a row, but the performance degradation seems to occur with any style change, and unrelated to the hover event handling. Attached is a very simple test-case with which I can consistently reproduce the problem. A couple of notes concerning the issue: Dynatrace reports show that nearly 100% of the CPU time is spent on "Calculating Generic Layout". This does not occur if <div> s are used instead of tables (see below). Switching the Document Mode to IE7 Standards or Quirks Mode via the Dev Toolbar fixes the problem. Due to constraints imposed on the environment I work in, IE8 runs in IE8 Compat Mode Browser Mode, with IE8 Standards Document Mode. Changing this setting through the Dev Toolbar doesn't have any effect on performance. Replacing the <table> solution with <div> / <span> approach improves performance, ruling out the amount of DOM nodes in itself as the culprit. The example adds mouseover events to each <tr> , but using event delegation doesn't mitigate the problem. In fact, if I replace the mouseover solution with a setInterval where every 50ms a random row is highlighted, the same performance degradation occurs. I have tested and confirmed this behavior on several different machines (all Windows XP, Intel Core Duo @ 2.33 Ghz, with 3.5 GB RAM) in my work environment. All exhibit the same behavior. I have tested HTML 4 Strict, XHTML 1.0 strict and HTML5 doctypes. All exhibit the same behavior.

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