Check if your pc have virus. You can download or maybe you already have some antivirus program. CCleaner is good, you can download advanced systemcare or you can buy it maybe. Clean %temp%, if you have less than 6 or 8 gb ram, end tasks you don't need. Adjust your pc for best perfomace or how you like it, you can also adjust startup programs as well. Laptop you can put on high performace, and if you have weird fps slowdown while playing game you can try put Max processor state to 99 instead of 100 (maybe could help). Boot your pc with max numb of processors. There is a lot of stuff you can do speed up your pc. I assume you game? So you should try few of these. If you have somewhat bad pc/lap download razer cortex and playing with setting in there. Or go to yt or google to see more tips/tricks.
Well, like I always wind up saying for all self-help threads, more info is required to pinpoint the nature of the problem.
When did your PC start slowing down, when does it slow down, what background apps are you running, how old is your PC, whats your RAM count, whats your OS, whats your shoe size, what have you done differently in the past 5 days....
Win10. I have plenty of experience with this. First off, unless you have a top of the line, $1000 PC, you shouldn't be running a browser and a game at the same time. Even a game that'll run on even the slowest potato like TF2. Secondly, if your disk usage is hitting 100%, thats normally a bug caused by some unnecessary services packed with win10. Unless of course you're doing the dumb thing i've been doing and letting the OWL streams run in the background for free tokens. lol. One thing that will DEFINITELY help with disk space is turning off Windows Search and Superfetch/Prefetch. windows search caches files on your system so it can find them faster when you search for them. Causes more harm than good. Superfetch/prefetch tries to boot programs it thinks youre gonna run next in the background to speed up load times. You can see why this is a problem. Both of these things can be found in Services. just do Windows Key + R then type in Services.msc. Another good thing to do while you're there is to set Windows Update to Manual so it can't automatically start an update download while you're doing something. just remember to go into your settings at least like once a month to check for updates.