This is a good question. People complain about how their phones lag after they've had it awhile and wonder why. I would say yes, but it depends on what apps the widget belongs to and how often they update.
As others have said, it depends on the phone. (How much RAM it has available to run things and how fast the processor is.) I have an Atrix 4G which is really power and fast. (I came from a Backflip, which was my first smart phone.) My Backflip kept running out of memory, even though I had a 16 GB SD card. I only had like 65 apps, most productivity apps, and about 12 to 15 widgets. That phone lagged with everything I did. On my Atrix, I have 22 widgets, 195 apps, can run my lapdock, which has two windows running, mobil and webtop view both at the same time, while my Atrix is docked and only see some lag every now and then.
We have to remember that today's cellphones are miniature computers and the more stuff we give them to do, the more they will lag, if the RAM gets really low or the processors can't keep up. Right now I'm typing this up on my lapdock, which is my Atrix, while streaming Pandora in the background and there is no lag in my typing at all.
So, know your phone and its capibilities and limits. Doing this will prevent future headachs as to trying to figure out what's wrong with your cell when it starts acting weird.