maybe your hard drive is too full, this will slow down your computer.
開一個網頁都要很久, have you check the internet speed or the connection you are using? if the wires are damaged/ the broadband company is reconstructing when you are using, the speed will be slow(not because of your computer)
It maybe your computer has virus, even though it is new, it can still get virus too, you should do a virus scan for your computer.
Or because you have too many 程式, but電腦是新的, that should not be a problem. But some 程式 may be not suitable for your computer, and make it slower. The common ones should be fine, just the programs that you installed yourself(like photo editing, games...)