I have been experiencing the exact same problem with my first Sim to own a business on a community lot. For me the lag is consistent tough and nothing seems to fix it not even the new update. I tried to move the household and put it back to the same lot (which I had moved a tiny bit). Game ran fine for about one Sim-day and then it lagged like crazy again and I didn't even visit my Sim's business on the community lot this time. If I take him there or anywhere else the game runs fine as well. It also loads super quickly - even after the lag occurred (no comparison to back in the day waiting minutes for a lot or neighborhood to load). My computer doesn't sound like it's working harder though.
I tested going to community lots with other sims and it worked fine. No lag. Same with businesses at home, also no lag.
I'm so sad because I was really looking forward to play TS2 again like back in the good old days. Hopefully it'll get fixed soon. I really wanted to try out the Build A City Challenge :(