I can suggest a few things to try that I have found makes a difference.
1. Your operating system and browser can make a difference. Has anything changed that you can remember? Try Google Chrom if you're using Internet Exployer or vis versa.
2. The advance settings can make a big difference. Go in and uncheck the 2 boxes, save the new setting and then close and restart the browser. This really helped me.
3. The time of day can make a difference. At home in the evening your internet could be slower with everyone home and connected with many devices. Even though the speed seems high it's the quality of the connection. I increased my internet service speed which seemed to help a lot. Your speeds sound OK but mine were about the same and going higher made a difference. I'm at 50/25 mbps.
4. I think Slingbox changes things quite often and sometimes they fix one thing and cause another problem. So it's important to use the forum and let them know what's not working. May be thing will be fixed after a few days. I've seen it happen more than once. Also, try calling them on the support line. They are very helpful.