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[same exact issue]

I'm not sure how I found your post but I'm glad I did.

I had the same exact issue and tuning the MTU/MSS did absolutely fix my issue.

What also helps is this link: https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000PN0gCAG

It covers the topic on setting the right tuning.  Hope it helps you (or another me in the future).