There are two reasons...
1st u keep opera running and minimise it when u launch other programs. then opera gets put to the pagefile and fragmented in the memory. thus when it regains focus it has to load a lot of stuff from ram and pagefile so it seems slow. best option other then a reinstall is to close the browser when opening other big programs like IDEs, games or for long periods of no use/ideal. anyways u can start it from the previous session so why minimise for long times.
second abt ur address bar taking too long to render keystrokes is mainly due to large no of history items. more precicely due to the sorting for giving u autocomplete feature. i bet u increased the history items at tools>preferences>advanced>history>addresses to a number greater then 500.
I keep it at 1000 so i get a half second delay while typing addresses on my laptop(celereonM 1.4GHz) and at 5000 on my PC(AMD@2.6GHZ).
clear this history and keep it low if ur processor is slow.
__________
another thing.. what u see in the task manager as mem usage is hardly correct. to view the actual amt of total memory used by a program keep it running and use for some time then open the performance page of the task manager here u have a graph and a bar to show Pagefile usage(total of pagefile and RAM) now note the reading below the PFUsage bar then close the program .. wait for the PFUsage bar to stabilize then just substract. I will bet it is much higher then shown in the processes page.
