I feel your pain, my PC (now 8 ish years old) has slowed down a lot over the last 6 months or so. I can't be productive at all.
I looked at off the shelf ones, which were decent enough but there would always be a compromise somewhere, like a weak PSU, integrated graphics, a low amount of RAM.
I soon realised that my £600 budget wasn't going to cut it, so I did my homework, starting just before Christmas and have now decided to build my own.
The main things for speed, as far as I can tell are RAM and using an SSD to run things, with (in my case) an HDD for storage. My main issue has been with my current PC, that it's pointless running 2 processing programs at the same time. If I load LR and PS at the same time, even when newer, it would affect my PC's performance. Those 2 bits of software are quite demanding on a PC's components, it is what it is but lately, it has gotten really bad, even browsing the web has become a chore.
Anyway, I've ended up going for a decent gaming spec, which whilst I don't game these days, it will allow me to do so at a decent level if I choose too.
new spec will be,
750watt, hybrid modular PSU,
32GB, DDR4 Vengeance RGB Pro RAM,
AMD Ryzen 5 5600 AM4, 6 core, 12 thread 65w CPU,
Antec A400i RGB CPU cooler,
1TB Crucial SSD (back up will be on 2, 2TB HDDs in different locations),
Nvidia 3060 GeForce OC 12GB, ray tracing GPU,
ASUS Tuf B550 mobo,
Antec NX410 ARGB case.
It doubled my budget but it will be overkill for me and hopefully, I'll be able to run at least 2 processing programs side by side as my RAM has jumped from currently 6GB to 32GB. I know there's a bit more to it but the new computer should certaily make PC life a bit less tedious. As mentioned, I can play the odd game now too, if I ever do get any spare time.