If you are having problems like I will outline below, check to see if you have the Asus ROG GameFirst app installed on your PC. UNINSTALL IT. The program was actually pretty resistant to being uninstalled for me, so you may have to delete the program file and then empty your recycle bin.
For the past 3 days I had been having issues with high latency ONLY in Classic. My ping would start off at a normal ~30 ms when I logged in. A few minutes later, it would increase to ~150 ms, increasing to ~300 ms and on from there at a steady pace. I did an absolute ton of troubleshooting.
I restarted my router; flushed DNS; released and renewed my IP; used a PingEnhancer tool; reset my UI; uninstalled and reinstalled the game; ran the scan and repair tool; did a clean boot; worked with my ISP for about an hour. NOTHING WORKED.
What did finally work was uninstalling ROG GameFirst, so give it a shot.