Can be many reasons. When you're using an "unlimited" plan it's likely that you're using a shared hosting, which means it's unlimited as in it won't charge you extra for more traffic or resource usage, but it will just slow down as it simply won't be able to serve the required connections. It's not usually advisable to run UNA on a shared hosting - you're likely to hit your memory limit pretty quickly and would also be affected by current load of other sites sharing the CPU resources. Any kind of heavier jobs like image transcoding or video transcoding would immediately oversaturate your CPU/memory allocation and even a single task like that could render your entire site unusable.