Only if your A-hole provider oversells their bandwidth.
Funny thing, when I sell to my clients, I can't say well, I'm charging you for 100 pieces, but I've oversold my inventory, so I'm going to give you 80 pieces and still keep your money.
I don't go along with that crap. When the ISP doesn't have the bandwidth to provide the speed it is charging you for, it should be forced to stop selling bandwidth. It's no less robbery than going into a gas station which charges you for 10 gallons and gives you five because it wants to sell gas to everyone even if it doesn't have the gas.
I don't know of any residential Internet service provider that provides access with any sort of SLA, uptime, speed, or otherwise.
If you want an SLA, you're going to have to pony up the bucks for commercial Internet service - and even then, it only covers access within the service provider's network, not bottlenecks in other nearby networks.