There is an option under Tools -> Options -> Engines -> General for "Enable support for testing Windows store applications". By default, that is turned on. Try disabling that option, restarting TestComplete, and see if you get a performance increase.
Disable Multiple Processes of Internet Explorer
Internet Explorer 9 and later spawn multiple iexplore.exe processes. This feature is called Loosely-Coupled Internet Explorer (LCIE) and is designed to improve the browser’s reliability and session isolation.
You must disable LCIE for Internet Explorer 11 on Windows 10. For other Internet Explorer versions, disabling LCIE is optional but can improve test performance.
To disable LCIE:
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Click Start and select Run. In the Run dialog box, type regedit and press Enter. This will open the Registry Editor.
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Find the following key:
HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main
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In this key, find the TabProcGrowth value.
If this value does not exist, create a new DWORD (32-bit) Value and name it TabProcGrowth.
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Set the TabProcGrowth value to 0.
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Close the Registry Editor.
This will instruct Internet Explorer to run the frame window and all of the tabs within the same iexplore.exe process rather than multiple processes.