Application startup very slow after boot

Apple iMac mit Retina 5K Late 2015

Hello,

i just got the latest 27" iMac (late 2015) some months ago with smallest Fusion Drive. Right from the beginning i noticed very slow boot startup and application startup which made me feel a little bit disappointed. I guess it was the first start of the fusion drive and also file vault which i disabled immediatly.

Now the speed is okay when the machine is working for some time. But after boot alot of apps don't start for a minute.

E.g. Skype starts but doesnt connects at all. I have to force close, start again and wait 1 minute. If i wait 10 minutes after boot and then start Skype it works fine. Another example is WebStorm (programming IDE) which doesn't load at all for 1 minute. Same here: If i wait 10 minutes or so after boot it would start immediatly.

Chrome starts immediatly(loaded from SSD?) but doesn't load any website for 10-30 seconds.

iCloud Drive was not usable at all. Sometimes it just hung for 10 minutes when i want to copy a file out of it(copy wont start at all, no error message or something). I disabled it and canceled my subscription (Google Drive works fine).

I already formatted and made a clean install of macOS Sierra. Also i disabled VileFault and iCloud.

Also i noticed (may not refer to this question):

Most of the times when i shutdown the machine there is a bar of pixel-errors at the top of the screen from left to right while the rest of the screen is black. Also sometimes when i wake it up from sleep (hours of sleep) it just shows this pixel-error-bar at the top with rest of the screen black and doesnt load macOS for minutes! Sometimes it shows the macOS login screen after 5 minutes, sometimes the display turns off after 5-10 minutes and if i turn it on again it shows the login screen immediatly. Image:

Maybe some of you have any idea whats wrong there. I know i can go to Apple Service Provider but 1. they may charge 70€ if they don't find anything and 2. its very unfavorable to transport such a big device without a car (public transport).