The "longer to start" could be low voltage to get her going or using alot of cranking time and drain because tractor wont start?

Also..with the taking longer to start, I would buy a few pints of Seafoam. Put about 6 ounces in oil and run for about 4 hours, drain and refill. As well, use 2 oz per gallon in fuel and clean out carb with the spray version. You may have gummed up needle and seat or linkage. With engine running (and hot), pour Seafoam in carb until it sputters and dies...if it doesnt die, shut it down ASAP. Allow to work in for 30 minutes or more, then restart.

I swear by Seafoam. That mower will run awesome and will start immediately.