@seafrog First check what @danfixesem suggested; The underside of mowers needs to be cleaned regularly. Both to make sure it's running right (no obstructions), and to prevent "rotting" of the mower deck.
The loss of power is also most often caused by incorrect air / fuel mixture. Seeing as it's a Toro, I'm assuming you're in the US, and I know that the US (and a lot of other countries) have problems with fuel from the pump, when used in small engine equipment. (This is because there's so many additives, especially Ethanol.) Many users have to clean their carburetors the start of every season (or get someone to do it for them), if the mower hasn't been stored correctly. Cleaning the carb on a mower isn't a very difficult job, but it can be tricky as the carbs have some incredibly tiny passages. The best results require an ultrasonic cleaner, and even they can't get it done sometimes.
I'd start with the following:
* ***Change the air filter (it should be changed once pr. season. It's crucial for not obstructing airflow) - This is #1***
* Change the spark plug (this has nothing to do with the loss of power, but should also be done every season)
* Change the oil on the mower (again, not related, but should be done once every 25 hours of use or once a season, whichever comes first). Most mowers require SAE30 oil, but some use 10W-30 or 10W-40