A bit late for the discussion, but since I found the thread...
In this moment, I'm forced to use Legacy mode, unlike before the current year, when I was using the Ultimate client for all the maps (except for the 500p one, which is unplayable with that client). Now, with my current PC (which it's relatively new), Ultimate client is hopelessly unplayable in all the maps: it has lags and crashes every moment, even with the Low Graphics option. Only it's possible to play that client in the mobile APP.
If I was using Ultimate client, but now I must use Legacy because whatever change made to Ultimate that became it unplayable, that says a lot about why do the community takes any attempt against Legacy as a personal daring. This remember me the paradox of the books, which are both the most backward and the most advanced technology in the current time. Likewise, I agree with Slyx when he says Legacy client is a relic from the past, but the issue here is: Legacy is currently, at the same time, technologically superior when compared with Ultimate.
Hey! It is indeed counter-logical that the game runs worse on a new machine. That shouldn't happen and points rather to a local problem. I am not aware of any recent performance degredation and also not of any noteworthy amounts of tickets in that regard. I myself also did not see any change of game performance in recently.
One thing that I can imagine is that on your new machine perhaps you onboard graphics card is rendering the game instead of your dedicated graphics card. Check your graphics card control panel (e.g. Nvidia settings) and check the program settings, which determine which program is rendered with which graphics card. Add the browser you run the game in to the programs you want the dedicated graphics card to handle.
Also check your browser settings if hardware acceleration is enabled. You could also try out a different browser just to rule out some causes. And make sure that your system and graphics drivers are up to date and no other program or browser tab is taking away too much computation time.