UserAgent: Mozilla/5.0 (X11; CrOS x86_64 10452.74.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.137 Safari/537.36
Platform: 10452.74.0 (Official Build) stable-channel peppy

Steps to reproduce the problem:
1. I'm not sure what this means
2. But, I factory reset mt Chromebook again and it still starts up slower than usual.
3. If I press the esc+refresh+power buttons a diagnostic page pops up telling me "Chrome OS is damaged or missing."

What is the expected behavior?
The expected behavior is that the Chromebook start up within five seconds.

What went wrong?
Today, I factory reset my Acer C720 Chromebook because I was no longer able to open PDF files, I believe I downloaded something unaware it would do this, but after the reset I was able to open the files. Shortly after the reset I received an update notification which I clicked on (the update was 66.0.3359.137). A few hours after that, after turning off my Chromebook to charge, the Chromebook instead of taking about five seconds to start-up it took about fifteen and continues to do so. I factory reset it again, thinking something went wrong, but that didn't work. So, after looking at posts from other uses I saw that if I held down the ESC, refresh, and power button I would be taken to a diagnostic page, which I was. The page said that "Chrome OS is missing or damaged. Please insert a USB stick or SD card." With a link underneath it for help. 

Now the "Chrome OS is missing or damaged" only pops up when I press and hold those three buttons, not when I turn it on. I did download google chrome on the Chromebook, but soon realized that isn't Chrome OS, so I deleted it. Therefore, my only problem is that my chrome book takes about fifteen seconds to boot-up, the fan still turns on within three to five seconds when the Chromebook is opened, however the screen that shows the Chrome symbol takes about fifteen seconds to turn on.

Is this because of the "chrome OS is missing or damaged," (which again does not pop up when turned only when I hold those three buttons), or the factory reset, or possibly the new update? Because just yesterday and this morning before all three of those things my Chromebook was working and turning on just fine as it always has been since purchased almost four years ago now, except not being able to open PDF files.

I hope this isn't a big problem and can be explained away easily, not that I'll need to buy a new Chromebook or do the recovery thing I've seen on other posts. Thanks for any and all help.

P.S. I wrote this post on the Chromebook mentioned throughout the post.

Did this work before? Yes The version before 66.0.3359.137 was the version I was using.

Chrome version: 66.0.3359.137  Channel: stable
OS Version: 10452.74.0
Flash Version: 29.0.0.140 /opt/google/chrome/pepper/libpepflashplayer.so

I all did was just pressing the combination of buttons (esc+refresh+power) and seeing that screen ("Chrome OS is missing or damaged"), and then hitting the power button to not be on that screen anymore. I did that three times, to see if it kept doing it, and it did.