Dust can build up and act as an insulator and cause heat to rise. When the parts get hot, they throttle to stay within a set thermal envelope and prevent damage.
Take the computer outside and using a can of compressed air or an air compressor and using short bursts, blow out the dust. Focus on the CPU and GPU heatsink and fan. Get the PSU really good as well, along with all fans and any filters you have. NEVER USE A VACUUM!
It's not a good idea to have it on the floor due to dust, carpet fibers, fur(if you have indoor pets), and static electricity. If it's on a wood floor, it's still a good idea at least put it up on something, even if it's just a piece of wood.