The OnePlus 6T runs on an older chip than the OnePlus 7 Pro — the Snapdragon 845 versus the Snapdragon 855, respectively.
Running on older chips sounds bad, but it's really not, especially with OnePlus phones. The older Snapdragon 845 from 2018 is still a workhorse. Coupled with OnePlus' lightweight Oxygen OS interface that runs on top of Android, and the fact that the OnePlus 6T comes with 8 GB of RAM, the OnePlus 6T is still an extremely capable smartphone.
In fact, the $550 OnePlus 6T now comes with 8 GB of RAM as standard, whereas the newer $670 OnePlus 7 Pro comes with 6 GB of RAM as standard. (If you buy the OnePlus 7 Pro from T-Mobile, the model with 6 GB of RAM isn't available. T-Mobile versions of the OnePlus 7 Pro start at $700 for the 8 GB RAM and 256 GB storage.)
For what it's worth, RAM is directly related to how fast your phone feels, and OnePlus phones have a lot of RAM. It's the thing that lets your phone keep the apps you recently used running in the background so that, when you return to those apps, they open almost immediately right where you left off. If your phone had no RAM, it would need to reopen every app, which takes longer.