Accross various Roms and kernels my arc handles a multitude of Widgets quite impressively. I typically have gmail, hotmail, a clock, extended controls, an animated speed dial app, pulse, weather, news all running and I don't experience any lag.
I do agree with the previous poster thatt live screens have a detrimental effect but hey if you want to have heaps of Widgets you don't need a live wallpapper you can't see do you.
Also with respect to loading applications I have never experienced anything beyond what would be considered normal load times. Add a slight overclock and you get basically no lag at all.
Sent from my LT15i.
Xperia Arc (Lt15I)
Relocked Bootloader, Stock .587, Stock Kernel (Given Away)
Xperia Arc S (Lt18a)
Relocked Bootloader, Stock ICS (Given Away)
Galaxy S III (GT-i9300)
Currently running: Sentinel Rom 4.70