My PC is a high-end laptop designed for gaming (I-7 and plenty of RAM, and a whoopie doo graphics doodad). Until the Showtime EP I never experienced lag unless I let myself get carried away with elaborate buildings and landscaping (many many varieties of plants/shrubs/trees on a single lot -- something we're advised not to do).
HOWEVER!!! After installing Showtime I've had nothing but lag. Generally, if I start a new sim family in a fresh world it will play fine for about six hours before lag pushes my patience over the edge.
SOLUTION. I've spent considerable time reading threads on the lag issue on a number of sites and this is what works for me. Delete the Showtime venues (Flying V coffeeshop, Kareoke place, etc). I did this in Edit Town using the bulldozer. In time, lag will commence being a problem again; when it does I type SHIFT+CONTROL+C and then enter "resetsim * ". This solves the lag problem for an hour or so, whereupon I go through the resetsim * process again. It's not a perfect solution, but it's stopped me from chucking the game altogether.
Shame shame shame on a company mature as EA for launching an EP so flawed, but then I'm quite certain they'll more than pay for it with the hit to their bottom line.