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The text is probably not the source of the performance problems. I would guess it's the imported raster images, if you are embedding them (rather than linking). You didn't say what your final saved format is, that you send to the printer. But if they need to be embedded, you could link them while you're creating the poster, and wait until right before you send to the printer, to embed them.
I'm not sure if this could be a source of the performance problems or not. But if you drag out a text box, with the text tool, but never type any text in it, the text box is still there, and adds data to the file. Theoretically, if you had a enough of them, they could be contributing to the performance problems. The only way I know to select them....well, now there are 2 ways to select them.
If you press the Tab key, it selects the first object that you created. Press it again, it selects the next one. Shift + Tab selects them in reverse order. So you can fairly quickly cycle through through everything in the file. By watching the status bar, you can see when an empty text box is selected. Note that the bounding box does not show up. But the status bar indicates it's selected. The status bar will say "Flowed Text (0 characters)....". Then just press Delete key to get rid of it.
The other way is using the (still relatively new) Objects menu > Objects. There you can easily see the text objects. If you click on a text object in Objects dialog, and you don't see any bounding box on the canvas, it's an empty text box, which you can delete. However, note that using the Objects dialog is known to cause performance problems. So don't keep it open all the time. Just open it when you need it, and close after.
Note that these empty text boxes don't get deleted with File menu > Clean Up Document.
If you convert the text to path, and it's a lot of text, that could also be causing performance issues.
If you'd like to share an SVG file with us, we could identify the specific source of the performance problems. Or else here's a tutorial which covers all the issues I know about.
https://forum.inkscapecommunity.com/index.php?action=articles;sa=view;article=35 (Actually, I should edit this article to include using the Objects dialog. I wrote it years ago, before Object manager was invented.)
I don't know why the Text tool works like this. But the first time you drag out a text box, it only has the one tiny diamond shaped handle. But if you grab that handle and drag it again, 2 other handles show up - a square in the top-left corner, and a circle in the top-right. They seem to work like the handles of the Rectangle tool. I don't know why, but sometimes I have to scribble around with the tiny diamond handle, before the other handles show up.
But you don't have to make a text box at all. When you drag out a text box, you start automatically typing Flowed Text. Flowed Text only exists in Inkscape. It won't show up on the internet, or in any other graphics program. Depending what your final format is, the flowed text might not be converted.
If you just click once on the canvas, with the Text tool, and then start typing, you create regular text, which doesn't have those problems. However, you have to use Return/Enter key at the end of a line. It doesn't automatically wrap. You can convert all your flowed text to regular text, by Text menu > Convert to Text.