The bloatware isn't going to slow the phone down, but some of it will use up a lot of battery and data.
Amazon Suites is the single worst app in that regard IMO.
I'm Verizon, but this is my list: ATT probably don't have most of these.
I have turned off the following on a Verizon phone:
Accessories, Amazon, Amazon App suite, Amazon Kindle, Amazon MP3, Appstore, Audible, BlurbCheckout, Bubbles, FamilyBase Companion, Flipboard, Message+, Messages, MyMagazine, NFL Mobile, Picasa Uploader, S Health, Smart Remote, Travel Wallpaper, Verizon Support & Protection (McAfee), VZ Navigator (pay for fee Navigation.... Google maps is free)
Google branded that I turned OFF:
Calendar Sync, Contacts Sync, Play Games, Play Movies & TV, Play Music, Play Newstand, Hangouts, Isis Mobile Wallet