> I'm currently using a Nokia 2600 Classic [1] with Symbian

There's no Symbian on that phone. Symbian was pretty much a smartphone OS that didn't really show up on feature phones. It had a full-blown WebKit browser, native apps, multitasking, productivity suites... you could even install Python interpreter and play with writing apps this way on Symbian phones from around that time (which is what I kept borrowing my mom's E65 for). Later versions even had Qt built-in.

Nokia 2600 Classic used one of the iterations of Series 40. Looking at the videos, it indeed appears rather slow on this phone.