A larger energy throughput needs a larger energy buffer to work, and a higher-throughput energy buffer has proportionally higher energy priority in Factorio's power system, so basically all of your power is going towards filling up the buffer before it powers other things again. This is why the factories all start out with low throughput limits, so the buffer doesn't break your factory early-game.
However in your case, a 50MW energy buffer needs ~1.6MJ of energy buffer with default settings, which can be achieved with a single steam engine in under 2 seconds. So either it's not a big deal as you make it seem, or you actually set your maximum power output to way more than 50MW, or you used the new "Power transfer update rate" setting, because the buffer size gets multiplied by the value of that setting. That setting really only should ever be used when you run into UPS trouble even though you've already performance-optimized all your other factory connections already, because power transfer is a pretty minor thing and it's usually more convenient to have legible power graphs and non-flashing lights than 2% more UPS.
