I've been using AE for 2 years and I'm working with some 4k footage for a project. I created proxies of the footage to make things move smoother, however my text keeps rendering pixelated. Even when my comp window is at Full resolution and 100% AND I turn my proxies off, the footage is crystal clear but the text is pixelated. Then, when exporting, once again the footage is clear but the text is pixelated. I know it's not an issue with the font because I've been using that same font for years...
Could this be an issue with the memory on my machine?
Purge the cache and all previews (it's in the edit-menu, I think).
If it's still pixellated, check all properties of the text layer (press uu to show all altered props and ee to show all effects and expressions). Create a new text layer and check if it's also pixellated and try another font if so.
Report the results, so we can troubleshoot further.