Blurry footage on timeline

When I add my footage to the timeline and have no video preview playing the still image is quite blurred. Now when I start playing the video it smoothes out and the quality looks good again. Rendering fixes this so the quality is always high, even when paused. 

If I add text to my footage and render the timeline again the text and footage become very blurred and the quality drops a lot. This shows in both the video preview and final exported file. 

If I go back and add new footage to a project that I worked on before, the new footage I add becomes incredibly blurred when rendered in the timeline. I made sure to check it was recorded with the exact same settings. This blurred footage also shows in the final product. 

Now I could choose not to render the timeline but that makes the video playback incredibly laggy and makes it hard to see what is happening on the screen. 

Is there anything I can do to prevent this from happening?