Slow client and low FPS for seemingly no reason

About a week ago, I tried opening League of Legends and found that while the top and side bars of the client loaded, the body remained black. All of the "buttons" in the top bar (play, collection, profile, etc) failed to perform the normal sound effect/animation when I hovered over them with my cursor, and each one took about four seconds to actually load into the client. Four seconds isn't a lot of time, but it's frustrating when I'm used to it opening instantly. Just getting into a draft queue felt like it took ages. Once I was in-game, my FPS (which is normally a nice playable 60) was hovering between 15 and 20 the whole time. It was frustrating, especially when all my settings were turned down to enhance performance. I've not recently done anything to my computer that would result in such low performance, and all the steps I've taken have done nothing. Obviously the first thing I did was uninstall/reinstall, which actually made the issue worse. I then went into my device manager and updated my graphics card. No change. I went into the League files and allowed the program to run itself in an administrative state, in case my firewall was blocking any necessary patch files. No change. I even disabled the in-game sounds, which I used to do to make the game work when I had a shitty computer. No change. I'm wondering what the next step should be, and if anyone else is having this issue. My computer specifications are: Operating System: Microsoft Windows 10 Home (64-bit) Processor: Intel® Core™ i5-5200U CPU @ 2.20GHz Graphics: Intel® HD Graphics 5500 Memory: 8 GB