i used to play on a laptop as well, about the same spec as yours (at least ram and gfx card) and i could play the game properly for a while, but after a while when a lot of guests and/or animals were in my zoo it started to slow down and it only got worse from then on. even with the built in fan my laptop became extremely hot (i could turn off the heating in my room, my laptop was taking care of that) so i would advise you to look for an external fan, there are some special fans for laptops, you place them under the laptop and they are connected by usb, i dont know any manufacturers but i think you could find one in a specialised computer store, or at least order one. without it the slowdown of zt2 is nearly inevitable, the only thing you could do without it is to close down your laptop, wait for it to cool down, then start playing again. one thing you could also do to give yourself more time to play before it slows down is to get rid of unwanted and unneccesary files and programs, free up some disk space. i came across this while playing another game, but it's the same thing, virtual memory uses your harddisk as memory, if your virtual memory is set to a higher value than you have diskspace left it wont do you any good. also the fuller your harddisk the slower your computer will run, so doing a little cleanup and a disk defrag (or whatever you call it) will work wonders, not only for zt2 but your laptop's overal performance. i hope you can find a way to keep playing, i couldn't, didn't think of the external fan thingy and didn't enjoy playing for half an hour then shutting down and wait for my laptop to cool. so i quit playing zt2 for a while, now i've got a brand new computer with the best parts i could afford and the game runs a lot better, but dont feel bad, even with my high performance computer, the game slows down over time. almost everyone has to face that at some point.
i hope i could help (i got a bit of a reputation of giving pointless advise and sharing thoughts which are already commonly known) goodluck and i hope you're able to fix the problem, if you do, let me know, because my little brother now has my laptop