If the "Persist display of execution time" option is enabled, when flipping through tabs in the Editor window it displays the same execution time for all tabs. The execution time displayed is not independant for each tab, it just shows the time for the last query executed no matter what tab it was run in. Toad should display the execution time for last query ran in that paricular tab, not just in any tab.
Open up a new Editor window to run a different query, instead of a new tab.
Waiting for fix to be released in a future version of Toad for Oracle.