Every hour of every day I have to restart Blender so it renders fast. Why?
…
WHY?
And will it ever be otherwise??
I don’t have to.
Solving this kind of things requires that you tell your GPU, driver version, operating system and version, Blender version and how to reproduce issue.
For me, it sometimes slows down too. But it is pretty much always quite obvious, because the materials get black, likely because it ran out of memory.
Edit: To clarify, this only happens when I literally work with scenes my computer can’t handle!
This is obvious, there is memory leak. Most likely it is not in operating system if not using some preview version, but Blender version or GPU driver may cause that.
First thing to do is update drivers, if using nVidia, pick some studio driver or WHQL driver. Latest is not always best. Also try Blender versions 2.83 LTS, 2.93 LTS and 3.3 LTS. LTS versions get patches longer periods so that is way to avoid bugs.
When you have tested working stack from Blender to hardware, you don’t need to upgarde things every day when there are new versions somewhere. You can keep things stable and working long time, it possible to standardize tools for years before changing production environment.
I’ve never once had this problem, and I’ve been using Blender every day for 3 years. I only restart my computer about once every two weeks, usually. I don’t think this is a universal thing
Sorry for not being clear. In my case, this only happens if I am going beyond what my computer can handle. If there is not enough memory, this is obviously going to cause issues.
To me it literally only happens, when I go past what I should do!
I restart my computer twice a year and I’ve used Blender from some version 2.x. I have not experienced this.
Couple of times I’ve triggered bug that Blender starts rapidly slowing when editing so I have to save file and restart Blender. Also I’ve experienced crashes.
I actually use Eevee when modelling and when using Cycles I use CPU, but when rendering on GPU using Cycles, I test run first on CPU how much ram is used and then I switch to GPU rendering. I believe modelling using Cycles and GPU may trigger bugs more easily.
This is simple too. If you run out of video ram it may cause issues and memory is not freed. So, don’t ever use VRAM more what you actually have. Also use some safe margin.
I’ve got only 2Gb of VRAM so I my workflow is to use CPU to do test renderds (whole scene), and switch to GPU when I’ve check that I dont use more VRAM that I have. Single asset modelling I do using GPU because I know it fit to memory.
???
restarting a computer cleans up all sorts of things, all living creatures need to sleep!
I have never understood this.
All living creatures need to sleep, braincells use that as opportunity throw out waste.
But computer is not living creature. However memory may get fragmented or buggy software may allocate resources and something is not released when they crash.
I have so plenty of memory that fragmentation is not issue. I can reboot my browser to get memory fragmentation resolved. Also it is extremely rare to have application to crash so closing them will release release everything.
That was my point, the operating system is a program and closing down helps clean thins up.
Operating system works well so I don’t need to do that.
The operating system does manage all the other programs and the entire memory system so in theory could be the route of any problem that occurs in any application.
It is only my opinion, if it works for you that’s fine. 
I was curious whether there was a useful answer in this thread, but unfortunately OP didn’t follow up (yet).
A quick look at the previous posts suggests that this is likely a RAM usage issue:
Not unlikely the purpose of posting was to let some steam off.
Andy,
what are the specs on your computer? e.g. ram, graphics card, OP?
Are you using Cycles or EEVEE? What version of Blender?
Never had that problem in with any version or any computer in 13yrs
since using Blender. 
