It sounds like you are expecting too much out of the computer.
You should write what components are in your machine.
Any windowed OS, whether from MS or Apple or Linux, can only handle a
limited number of open windows. Performance will also slow depending on the
amount of RAM in the computer and the CPU speed or number of cores.
How "slow" a computer seems is often subjective: after moving to my uber
fast quad core with zillions of GBS of RAM, a 64 bit OS and a high end
Radeon video card I was quite impressed with the speed of the machine. Now
after a few months of daily use it seems slow . . . .