I often have a similar issue, which is importing PDFs exported from CAD, Vectorworks and Sketchup, architectural drawings. I don't have a good answer, but what I do is run through the drawings and select lines and dimensions I absolutely need, then paste it into a new document. Sometimes I select all white and unstroked unfilled objects and delete them at once, which can help. Generally I'm less and less impressed with Illustrator's performance. Photoshop gets faster seemingly all the time, but Illustrator isn't noticeably faster than it was ten years ago. Big complex documents just kill it. Interestingly, I've just downloaded Affinity Design, and early indications for me are that it doesn't appear to have the same problem with large complex documents. In one complex Illustrator file (3,000 paths, 13,000 points, 35mb), moving one object incurs a delay of 1-2 seconds for me, while no lag exists in Affinity Design with the exact same complex object in the exact same file.
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