This is something that I've noticed since Windows 8, but Windows Defender make some programs so slow to open. It's pretty noticeable on the programs that it does it on as it takes approx. 20-30 seconds to open them, even from an SSD, and if you look at Resource Monitor listed by CPU usage, you can see it's due to MsMpEng.exe running a scan before the program will open.

It would be understandable if it was doing it the first time the program had ever ran, or if the program wasn't signed, but that's not always the case. Canon DPP for example isn't signed, so presumably that's why Defender keeps scanning that, however here's an example of another program (SyncBackFree) that is signed, but yet Windows Defender still wants to keep scanning it on a daily basis despite nothing changing since previous scans.

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Obviously I can add individual programs to the Windows Defender Exclusion list and exclude them from real-time protection, but what it is that Defender doesn't like with certain programs that is feels the need to keep scanning them daily? Is this a Microsoft problem, or is it down to the software not adhering to some sort of best practice policies Microsoft have laid out? There must me something that makes Defender treat it as potentially signed malware?