In general, when people talk about their computer being slow, I mention dust. As a former computer tech with 15 years of professional experience, I found that simply blowing out dust can significantly improve performance.
I'm not talking about a thin, almost imperceptible amount of dust, but rather large clumps or even mats that prevent normal airflow. I've seen heat sinks that had basically a filter over them that was dust, rather than an actual filter. This blocks a very significant amount of air from ever cooling the CPU. Removing dust like this will tend to quiet fans instantly and allow your components to survive longer. Heat had killed many a computer I was asked to fix.
Going along with the heat issue idea, you might also try better thermal paste. The white cr@p most processors come with is like the Yugo of thermal paste. I use Arctic Silver, but there's better stuff than that, even. Arctic Silver is about a Porche (using the car rating scale), but there are Ferrari's and supercar varieties out there.
Processors tend to slow down when they are overheating. This is a physical thing as well as a "self preservation" programmed into many CPUs. I don't know if it'll still show 100% on the Task Manager or if it'll show 40% (like you see), but it can be a significant slowdown while the CPU tries to let the heat sink and fan "catch up."
Another thing that could be slowing your CPU down is the GPU. If you are running graphic intensive games or utilities (like CAD), your GPU might be holding back your CPU. Getting a better video card might be something to look at. Also, using the wrong right card might be holding you back. Gaming cards aren't (usually) designed to work with CAD as well as workstation cards, and workstation cards (usually) won't game very well, either. Some do, but most don't.
As @Jaroslav Kucera mentioned, it could be disk related. Hitting the HD(s) a lot can slow you way down. I normally run multiple drives. One dedicated to the OS and other(s) for software, Windows page file, personal files, etc. Besides not having to worry so much about backing personal data up in case of OS failure, having multiple HDs spreads the workload considerably. Reading and writing to the same disk at the same time can seriously slow down the HD. Using SHDs can mitigate this, but not entirely. Photoshop and video editing software are known to hit HDs hard. Reading from one HD then outputting to a 2nd HD is the way to go. This also helps the life of your HDs. I also go with active cooling on my HDs. I haven't killed a hard drive since I put a fan and heat sink on them +15 years ago. Google them, they are cheap insurance.
Believe it or not, your PSU might be slowing you down, too. If you don't have enough power (or your PSU is old or a cheap, over rated, Chinese POS), you can have performance issues. I've seen first hand what odd OS issues a flaky PSU can do. You are looking for voltage as well as amps, so make sure they all match the specs on the PSU, if you go this route, and also make sure they meet or exceed your power needs. If your components total 500 watts and you're giving them even 475, that's bad. I recommend going over your requirements by about 20%, so as your PSU gets older (and drops power) and your other components get older (and require mote power), you aren't stuck buying a new PSU so quickly.
Including the other answers here, there are still more reasons for your computer to run slow. Except for the PSU option, what I talked about were very commonly seen when I was a computer tech. Doing a benchmark and other tests are the only way you'll be able to figure things out. Swapping parts might not even solve the issue if it's a combo of multiple parts causing the slowdown.
And, AFAIK, there's no way to force your computer to use 100% of the processor. The CPU and OS knows what they need to do and are really good at their jobs, usually. :-) I don't think anyone has yet figured out a way to force feed a CPU to make it run at 100% when you think it should. At least not without feeding it extra junk to make the percentage "look good."
With you seeing 40% and not and a whole number division of 100% (like 25%, 33%, or 50%), I have a feeling it's not a single threading issue. It could be, but that's not where my mind goes. +1 to @Twisty Impersonator for bringing it up right away, though.
Good luck trying to figure this one out! I've spend days trying to figure this kind of thing out, only to end up replacing most of the guts as a "last resort."