looking to get an affordable boat for bass fishing. I'm just wondering if anyone has any experience with 2.5 hp outboards on a 10-12 foot jon boat. I'm not looking for tremendous performance, just wondering roughly how fast it would go with 2 people and minimal equipment on board

I would expect between 5 to 10 mph. 

I have a Mercury 2.5 on my 14 ft sea scamp. Loaded to capacity with a tailwind it’ll do 6 mph. 

A large TM will make the boat run about the same speed as a 2.5, the big advantage is the gas motor is a whole lot lighter and you don't have to worry about the battery going dead after 30 minutes of full speed.   It beats the hell out of paddling, but don't expect to get there in a hurry.  I have a 4hp Merc I used on my 17' canoe.  Now that can get scarry if you try to turn at WOT.

It is nice not having to lug a battery or an external gas tank. Mine’s a four stroke so I ain’t even got to remember oil. 

It's hardly a motor.  I'd spend a wee bit more and get something that can get the boat on plane.  I spent the first 20 years of my life fishing out of a 10' boat with a 2hp or 10hp motor (uncle left the 10 at the cabin sometimes) and the difference was monstrous.  With the 2 it wouldn't do 5, but when my cousin brought his 15hp it would do more than 30.  The 10 was a sweet spot for the boat though and rated HP.  Whatever that hull will take I would do it.  Older low HP motors are reliable so make up for the size difference dollar wise with age.  At least that is what I would do.

Yep. I have a 9.9 hp Johnson that’ll make that baby boogie. If you’re planning on using it on a big lake a 2.5 would be frustrating, but for ponds and small backwater it’s the berries. 

Different 12' Jon boats...my current 12' is only rated for 3 hp, my 2.2 does a good job but about 6-7 mph is all the speed you will get, probably closer to 4-5 mph with two adults.  In years last I had a 12' " river jon" that was a wide deep Jon boat rated for 10 hp.  With my OMC 9.9 it would fly, right up on plane with two adults over 20 mph.  The boat needs to be built to handle a bigger motor, putting a 9.9 on a boat rated for 3 to is an accident looking to happen !!  When it does, I am sure it will be someone else's fault !!