I like @wpns's answer but to add:
Most likely causes to slow performance:
If too much CPU is utilized, resources for processes will be scarce. You say that you don't see any process using 100% of CPU but is the CPU 100% in use? Lots of small tasks could collectively eat up the CPU.
Security applications like antivirus are frequently a cause for performance problems. Take note if you have scheduled scans during the times you have issues. Try to schedule scans for periods when you're not using the machine. Some applications allow you to temporarily disable their features which would allow you to confirm if they are the problem. I would take care in doing so, you may wish to disconnect your machine from the internet before disabling security features.
If a CPU gets too hot, it will also deliberately throttle down the performance (at least chips made since around 2002). If the machine is getting warm from the environment, or the fans are not working well, or the heatsinks are clogged with dust, you could be experiencing performance issues from throttling. There are a number of utilities that can read the temperature for the CPU, some BIOS will also report this information. Appropriate temperatures for your machine will vary wildly based on design but I'd say anything above 50-60C would be too hot.
If a machine doesn't have enough RAM available, it will begin to use more and more Virtual Memory which is when hard drive space is used for memory. Hard drives are much slower than RAM and this will cause the machine to slow. The more RAM is used, the more this will begin to affect your performance. A computer will try to optimize virtual memory to reduce access and put more frequent items on real RAM and less frequent in HD. An access to virtual memory is called a 'hard fault' and the number of them can be seen in the task manager. Higher numbers of hard faults are bad. You should not see the number above 10 in normal operation.
It's worth noting that your computer may seem like it has a lot of RAM available but Win10 uses RAM in 'special' ways. Greater than 75% usage of your system RAM will trigger special conditions on your machine. You can also see the 'real' RAM usage under 'commit' in task manager, the number will often exceed the physical RAM in the system.
As @wpns said, applications frequently have tiny errors that can 'waste' RAM over time. A reboot of the system will usually resolve that issue. If you're experiencing a daily problem, your issue is more severe than average however. Take note if you have this problem more often with certain applications.
- Hard Drive
When a hard disk performance falls, this can make virtual memory issues worse. However, a hard disk issue can be bad even when RAM is available. Certain applications and portions of the OS are designed to read from HD under normal conditions. This can cause slow performance if HD performance is bad. A failing HD will often cause very long delays. MS Word will automatically save your working document to recovery every 10 minutes by default. When this occurs, you could see long delays before your system responds if HD performance is bad. Use a utility such as SMART or CrystalDisk to examine the performance of your HD.
Extra applications, intentional or unintentional (adware, spyware, malware), can cause 'hidden' delays on your system. Sometimes it's not possible to see an application's impact from task manager. In some cases, the operating system itself can be corrupted and cause problems.
You can diagnose some of these issues by putting your machine into minimal start (using msconfig), safe mode, reinstalling your operating system, or running a temporary live boot cd (knoppix, ubuntu) and seeing if your machine behaves similarly in those modes.
I once had a machine that would refuse to run correctly with Windows installed but ran fine with Ubuntu. I never figured out why and decided it wasn't worth my time since it was just an internet browsing laptop.
Best of luck with your search!