iMac is slow to boot

I have a Retina 4K iMac (3.1GHz i5, 16GB RAM, 1TB Fusion Drive) that is only 3 months old. About a month after I got it, I installed Windows on it, but I noticed that OS X was taking significantly longer to boot up (originally about 10s to now over a minute), I thought this was to do with Windows so I didn't think much about it. However, after Windows caused me a few problems, I decided to uninstall it (I used bootcamp to both install it and uninstall it). Boot up time was still long. As most of my important data is in the cloud, the only issue with reformatting is just that it's annoying to reinstall the apps and get the settings the way I want so I decided to format the hard drive in Internet recovery mode. After reinstalling however, it's just as slow. Not only is it slow, but the loading bar moves fast for the first half and then slows until it stalls, then the loading become a fat white box until it brings me to the login screen. Something weird I found that I think could be related is when I hold option at boot, there is still a Windows option. I decided to see what would happen if I pressed it: it brings me to a screen saying "Recovery, Your PC/Device needs to be repaired". Why does it still show (obviously the data has been wiped even if traces of the OS are showing) even when I formatted the drive? I have already tried: resetting the NVRAM, resetting the SMC, making sure the disk I want it checked in startup disk. Only a USB keyboard and mouse are plugged in during startup. Why is it so slow (sorry that this is so long)?