I too have that particular SanDisk Cruzer USB brand and 16G size which I probably bought because it was the least expensive at Walmart at the time. I used it successfully a few years ago, but took it off as it's use cancels the ability to switch between two channels with the remote button, and I like to switch channels to avoid the commercials. So, this week I hooked up the USB again and it got the "too slow" error message even after two or tree tries to get it to work. Then, I waited a few hours and tried it again and it worked normally this time. I'm not sure the "too slow" message is related to internet wifi speed or just a glitch. But, I'd suggest trying it many times at different times of day and see what happens instead of buying a new one. And if you want the feature to switch between two channels just remove the USB temporarily. Video demo of this: https://www.youtube.com/watch?v=a1SES2wSGZ8
I know this is an old thread, but recently ran into this on a new USB. Take the USB out and use your computer to format the stick to NTFS. You have to re-format it again on the Roku, but it should work.