A computer will often slow down a little once all the software you want is loaded. You will notice increased boot times and load times, but all of that is usually completely normal. The reason is that all those background tasks and software are using resources. They are using memory, CPU cycles, and drive bandwidth. This will make a PC seem a little bit slower even though the hardware hasn't degraded at all. I always tell people to load up EVERYTHING they want before they benchmark, otherwise they set an impossible standard for their PC to live up to.
As for your frame rate anxiety, turn the FPS counters off. Play the game by feel. Gamers in general are getting too wound up in PERFORMANCE of the game like their FPS is their high score. If you are playing the game for entertainment, then focus on the game and have fun. If you are playing the game just to see what kind of performance you get, then you are doing it wrong. Partially the tech media is to blame for this. They want quantitative numbers to rate hardware. There is simply more emphasis put on frame rates than feel because it is harder to describe a feel than it is to state a number.
When Ryzen first launched and it was up against the 7700K, people tried both systems and many said that the Ryzen system felt, well, better. AMD had the frame rate counters turned off in these demos. When the hard numbers came out and the max FPS was still on Intel, AMD was discounted as being not as good, however, AMD had more consistent frame rates and the minimum FPS were higher, and this led to a better feel with the game.
To help with feel, more responsive inputs, like a gaming keyboard or mouse, can help. Also a high refresh rate monitor will make things feel better too, especially one with GSync in your case. Your computer doesn't need to be any faster to improve your experience. Lots of people with far less impressive hardware have a better time because of these quality of life improvements. A smooth and responsive game is much more enjoyable than sky high frame rates, and is what everyone should try to get.