Had the same type of problem. I the past while playing TW1 I always used Opera as the browser of choice bc it allowed for arranging tight trains & other advantages over IE & Firefox. My preference is for IE bc I trust the security measures Microsoft has in their programming. Fast forward a couple of years and I am back trying TW2 with a newer computer running Windows 7, not XP as before and an internet enabled version of Norton Antivirus.
My computer had been working fine before I started the game. The first couple of days I played with the different browsers to see if any of them worked better than another, IE, Firefox & Opera. I found that they all worked about the same on my machine but anticipated that I would need to use Opera for the same reasons as TW1. A couple of days into the game I noticed my cooling fan is racing continuously, even when the machine is idle with no programs running on the desktop,which was never an issue before and ALL my programs are severely lagging. Everything from opening & closing programs to shutting down the machine are running 30 seconds to a minute or more behind the command clicks. I checked the task manager performance monitor to find my processor working at 80+% and my RAM at 70+%. Normally my processor is less than 40% and RAM less than 50%. I started watching the processes and found that my Norton antivirus is running constantly, eating up my processor space. I've had the Norton since the computer was new 2 years ago and never had this problem so I don't think that is it. I worked backwards uninstalling the most recently installed programs & restarting the machine to test after each uninstall. There were only 2 new programs I had installed in the last several months, Google Earth & Opera. Once I uninstalled the Opera and restarted, the problem was cured. I'm not a computer pro, but it acted like the Norton was constantly attacking the Opera. I am now using Firefox and my CPU is running around 30% and RAM running at about 50% with the game running. Lag no more! Hooray!
