Cyan Cooper

How is this in any way an answer to my question?

Attached 😊
I-485 Affinity File.afpub

@Pšenda You rock man, thanks for the tip!

Encapsulate these layers in a group. This limits the effect of the Live filter to only the layers in the group.

Thank you for clarifying! I removed it and the white border on the exported file went away.
It's a little inconvenient that you can't use a live filter layer without offering it to specific layers. Hopefully Affinity's product team looks into this!

Problem is in this layer (Blur live filter).

Must be deleted (or set to 0px), not only disabled.

I used the word "sin" in jest. 
If one were to seriously promote Affinity products, it would take an investment of time and possibly money. I see nothing wrong with affiliate programs. They're simply another advertising channel. If Serif chooses not to offer one, that's fine. But I don't feel our motives should be judged when we request one, as has been done by some in this topic.
There's a big difference between casually recommending Affinity products vs putting effort into a marketing campaign to actively promote the software. I don't recommend products as an affiliate unless I believe in them, so that isn't an issue for me.

I have this issue too! New 2019 iMac:
iMac (Retina 5K, 27-inch, 2019), 3.7 GHz 6-Core Intel Core i5, 40 GB 2667 MHz DDR4, Radeon Pro 580X 8 GB
It can run After Effects and Final Cut Pro no problem, but the second I start doing things in Affinity Publisher activity monitor shows the software taking 500%+ of CPU.
As you can see in the image, the composition I'm working on isn't exactly complex, and every single element was created in Publisher (nothing imported from Photoshop).
Any help on this is much appreciated. We've built our business on Affinity products—which work fine on our laptops—but if we can't use them on an iMac without the computer seizing up then we'll have to figure something else out.
 

This just saved my butt, thank you!

Hi RachelMCD,
Welcome to Affinity Forums
Without seeing at least a screenshot of the application's interface with the text selected and Layers and Colours panel visible  is difficult to guess what may be happening. If you could attach the .afdesign file it would be even easier to help you.
Does you have applied any transparency to the text? With the text selected, click the Transparency Tool (the cup of glass icon) and check if the Type dropdown in the context toolbar is set to None. Does the colour you selected for the text has the opacity set to 100% (on the bottom of the Colour panel)?