When did you notice a drop in quality?

In Season 2, when I realized that they were just rehashing the season long story arc from Season 1.

Really, this show would have worked better with the first season being a 12-part mini-series and the rest of the seasons being not at all made.

It's had its ups and downs. I thought it was pretty good until this season.

I liked Arrow seasons 1 through 7. Something I noticed is some times Arrow would have a baddie he has to fight that he actually triumphs over before the season is over and then a new one steps in the limelight to be the seasons "big" enemy. I enjoyed that change of pace. Its nice to have bad guys come into focus and leave focus throughout the season than to focus on a single enemy the whole time. Because then you know they arent going to defeat that enemy until the seasons final episode. This is why Flash became tedious for me, is because it always had one major villian that was in focus for that season and they only triumphed in the final episode of that season.

To answer your initial question, I stopped watching Flash after episode 2 of season 6. I just lost interest. I got tired of the premise and began noticing more things about situations and characters I did not like. You nailed it with "too many people just hanging out in star labs" it basically became a friendly batcave. Also the intelligent characters were used for many situations that made no sense. For example Caitlyn Snow was helping Cisco hack into a computer...WHAT? Just because the lady has a doctorate in physics doesnt mean she would know jack squat about computer programming!!! At least in Arrow, Felicity Jones was a computer programmer and thats why she knew how to hack. But in Flash you'll notice that if a character is "intelligent/genius" they suddenly can do any "intelligent" task that the scene requires. NOPE. When people get a masters degree in something, they are focusing all their time and energy into that particular field. So they dont have this all around genius knowledge on everything just because they are a genius. Thats why people with multiple doctorates are rare. It takes a lot of time and energy to obtain all that knowledge. This is like when theres any tech in Marvel they just say the magic word "nanoparticles" to explain away how it just works. "Genius people are gonna be genius".