This worked for me, had to do a bit of trial and error but I think leaving only cpu 0, 1 and 2 works the best for me, i7 7700, uninstalling AV probably works too but its temporary, I did it before
I7-3770K/GTX 1070/16 GB RAM…this stuttering in this game (Netflix in Google Chrome as well) had been driving me crazy trying to figure it out, so much that I even reinstalled Windows. Ran well enough, but came back shortly after Windows completed all of its updates. Sure enough, after reading here, I uninstalled Avast, and the game runs great!
Netflix stutter issue is still present, and appears to be an issue with the Nvidia drivers and Google Chrome - not present at all with an old ATI card I have lying around.
THANK YOU to everyone who suggested it was an antivirus problem!!!
Can also confirm removing Avast completely fixed thanks so much! Time to get playing 
Very interesting how the game started to stutter for many people recently.
I was very confused because for me it started happening exactly when I switched from an R9-280X 3GB to my current RX 480 8GB. With the 280X and 18.x.x driver I did not get any stuttering on high settings in FM7, and then when I tried the game with the RX 480, a much better GPU, even on low I was stuttering.
I thought it was the new driver, so I reverted to 18.x.x (can’t remember which exactly), but it didn’t solve the issue.
Interestingly, removing Avast did solve the stuttering. I’ve been using Avast for 3-4 years and never had any problems with it, any other game also ran fine with Avast installed (including Horizon 3 and 4).
Uninstalling Avast also worked for me.
This has only been an issue for me since the last Windows update. I’ve been using Avast and running FM7 for months without any issues.
What worked for me, after months of frustration and troubleshooting was, I seen another user mention it working fine on a HDD but not on a SSD.
So I removed the game and all dlc’s from my SSD (Crucial MX300 M.2) and set it to install on my HDD (WD Blue 1TB) and its been running butter smooth 144 fps for last month or so. not sure if its a crucial thing or an m.2 thing or an ssd thing.
But it works great on my HDD, loads a little slower though, anyways I hope this helps someone as it solved a lot of frustration for me.
I have no anti virus, i have a very high end pc yet i can’t play any damn forza without stuttering and poor performance. Absolutely beyond the joke that i can’t run any motorsport or horizon on my pc without really poor performance, i’d like a real solution not these uninstall your anti virus which is a load of crap.
This fixed it for me, reinstall windows 10, reinstall all drivers, reinstall forza 7 (I know, 100+gig download, very annoying) and run without a anti-virus. Now it runs super smooth.
This solution takes a lot of time and bandwidth so only recommended if you cant fix it any other way.
HAHAHAHA good trolling.
Im having severe stutter with i7 7700k GTX 1080 16gb Corsair vengeance DDR4 LPX.
As a Gran Turismo fan Its nice to see what a -fine- piece of -software- this game is 
I have stuttering too. From game relase. Bigger, smaller but still present. Recently even more often. Its really bad. Sometims I can drive few races quite without problem, but yesterday every few laps 1 to 3 stutters.
I have Windows Defender. GTX1080Ti, 16GB of RAM, Intel Core i7-8086K, game is on SSD - I havent got HDD now. Windows with latest updates. Every other game work like butter (Asseto Corsa, GTA V, Battlefields etc.). Only Forza 7 stutters. 
Sometimes clearing cache in Forza User folder helps for a while…
I will check that CPU issue. Maybe HT is a problem, and game should operate only on physical cores not with HT. I will check this.
rtx 2070, ryzen 5 2600x, 16gb ram
Had the exact same problem. Micro stutter/freezes. Most noticeable on Dubai.
The game was installed on my crucial ssd. After moving it to my old hdd the problem is completely gone.
Thanks for the tip.
I lied. Problem is still there but much less. Moved it back to my ssd just to be sure and stutters became more frequent.
So less micro stutter on my hdd but they are still there on some tracks. They also seem to last a fraction longer when they do still happen.
The fragility of this title is worrying!? It seems that any back ground processes is the enemy.
I’ve just built my first PC for years, sold my Xbox X (because it was made by accountants and it was as stable as the Titanic), Drifting since Forza 3 (not Horizon 3!!).
Now, nothing bbbut sssstuttering!!
C’mon Turn 10??
to each patch els deteriorate the graphic quality of the game. very disappointed with this
I run:
Gtx 1080ti
Ryzen 2700x
Samsung 850evo ssd
16gb of 3600mhz ddr4
4k 60hz monitor
2160x1440 144hz gsync monitor.
I’ve tried it all.
-Updating graphics drivers,
-Updating windows,
-Trying to update forza,
-Turning my graphics down,
-Turning everything to dynamic
-locking to 60fps
-turning off gsync
-turn off windows game mode
-disable other monitors
-close all background programs
problem persists.
from what I’m seeing…my cpu and gpu are just cruising with no issue during the studdering but something is absolutely pinning my network. The fact that fm7 on pc is online only drm is bullshit but that’s another conversation. I feel like it’s trying to upload-download data constantly, perhaps even live track data, which may be causing the studdering. This used to be my favorite game to play and this instability is just ridiculous. this issue has been constant for nearing a year now, and monthly updates later, still no improvement.
Forza Horizon 4 runs perfectly fine with every setting pinned at 100fps with zero studdering. I’m not going to pretend that this is an easy fix for turn10, but I’m beginning to question whether they’re actually putting resources into fixing this or just dumping everything they have into what is likely going to be forza motorsport 8. releasing a new game is NOT a fix.
Same here!
The stuttering only happens on multiplayer…
So the cpu affinity thing made it much better but still not perfect so I decided to uninstall my AV again, maybe I will get a few more months of smooth gameplay…nope, 2 weeks later its back to stutters again, really, effin really?!?
So, got a new monitor, 1440P 144hz with adaptive sync. And yes, had mayor stutters again…until I locked the game to 60 FPS…smooth as butter. It seems this game is poorly optimized to run higher then 60 FPS. My computer runs the game when FPS is free on ultra between 90 and 140FPS, but I get stutters a lot when I run the game above 60FPS, v-synced, free-synced and without sync on.
To bring to attention of Turn 10:
POSSIBLE INTERFACE PROBLEM(S) BETWEEN FM7 & THE XBOX APP FOR WINDOWS 10
I have experienced severe stuttering in FM7 PC caused by the Xbox app while in multiplayer. It has happened when other players attempt to send me a message via Xbox app, regardless of whether in lobby or during a race.
After EVERY instance of stutter I have experienced that was not accompanied by the dreaded “Low Bandwith” or “High Latency” message in-game, I have exited FM7 to find a pop-up message notice from the Xbox app, with a timestamp indicating said msg was sent while I was still in-game.