Awww. Sorry to hear about that, Pink
Since the new client, I seem to have more lag...but it's tough to really gauge the cause of the lag since there's about a bazillion more people on game than before the client.
I can hang out on the main map for a fair amount of time, and will gradually bog down the longer I stay there. Eventually it's almost a stop/start type of movement where everything stops for a few seconds, then starts again and a bunch of chat bubbles go up at once.
Not a new problem for me, but for quite awhile...if there's a quest that takes me into Central Square, I'm almost guaranteed to crash...even in single mode! Tonight it was happening again. My workaround was purging the browser cache in between logging on the next 'family member'. No problems after that...well, except the blech hunt itself.
Not sure what's causing your problem. There are many factors that could make it worse: age of your computer, how many programs running in the background, etc. Single mode sounds to be your best option as the others have already said. If your computer is older (3+years), or an 'entry-level' computer that is not quite that old, it might be time to start saving for a new one. If you're using Internet Explorer, try switching to Firefox.
Hope it improves for you. In the meantime, have some coffee and a donut...take a step back...and try approaching it again when you're ready
Flamanar