Spyndel is absolutely right and I think he gave you the best possible practical advice. ZBrush works well even with modest and old computers, so I think your modern CPU should be more than enough. Actually it’s a very good processor.
If ZBrush is really slow I think the problem must be somewhere else. Maybe insufficient RAM? scratch disk too full? But I don’t think so either… Maybe something we can’t even imagine, there is a million ways things can go wrong.
Perhaps the issue is not related with hardware or your OS … Having a fast PC sometimes makes people fall into the temptation to subdivide (or increase dynamesh resolution) way too soon. For instance, if you have 12 million polygons and try to use the move brush with a very big radius size to block the main forms and shapes you might experience lag but this is normal, you are moving millions of polygons at once with simple brush stroke when at a lower subdivision it would be maybe a hundred. Generally speaking, if you reached 16 million polygons, you should probably be working on surface details and not on the big shapes. As Spyndel said, maybe " you are expecting too much performance out of too dense a polymesh". Using move elastic with a big brush size on a dense mesh will always be slow.
Now, it’s possible that you knew all this and just want to talk about PC components and what’s the best possible Rig for ZBrush…
Right now I think the best CPUs for ZBrush (inside the human affordable range) are probably the new AMD Ryzen 7 since ZBrush uses multi threading and these Processor have 8 cores and 16 threads. If you use Keyshot or another CPU based renderer all those threads will make rendering much much faster. The problem with this is you will have to change the motherboard as well so, if you just want to upgrade your system, maybe this is not an option for you. Without changing your Mother Board you can only upgrade by buying a Intel 6700 or 7700. I would go for the 7700 for 350$ and since you won’t need your CPU you could sell it.
The Ryzen 7 1700X has a great value but it’s still a 400 $ CPU, 8 cores but not cheap. AMD will release Ryzen 5 soon and they will be cheaper, 6 cores and 12 threads for around 250 $ for the 1600X. That sounds really good and you can find ASUS mother boards for ryzen for 100 $…And it seems the Fans AMD is including with their CPUs are great.
If you could sell your current mother board and CPU for, let’s say, 200/ 300 you might not need a lot more to upgrade to a ryzen 7 (400$ for 1700x + 100$ ASUS Mother Board). That is if you can fit the new MB in your case, use your current PSU, RAM,GPU etc… That should fit within your 540$ budget and even have some extra money for more RAM.
Also, I believe (but I am Not sure of this at all) that your windows 10 installation might be attached to your MB and it might not work. You would need a fresh installation and maybe even buy another Windows 10…
Now, It’s fair to say that I don’t own a Ryzen system and all I am saying is based on users reviews and benchmarks. Maybe some Zbrush users are already using Ryzen and want to share how it’s working for them. I have a i7 with 6 cores, I am happy, it is plenty for what I need etc… but I would totally change it for a Ryzen 7 if I had to build it today. I don’t think I would see any difference in ZBrush Performance but It would be more noticeable at rendering.