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To reproduce try to open a bigger html file (> 100kB). The file opening includes the navigator initialization which recursively expands the whole tree of nodes representing the tags in the code. This is very costly operation and needs to be avoided in any case. IMHO expanding just first one or two levels of nodes would be enought. This is not HTML issue in fact, the logic is coded in GSF implementation, but for HTML with its big and deep tree the problem is most flagrant.
*** This issue has been marked as a duplicate of 132991 ***
v.