The game lags significantly when there are too many particles rendered at once. This issue will apply to anything that creates a large amount of particles including nether portals.
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Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-18171.
Your additional information and observations are very useful, so if you can add them to the comments section of the parent issue it would be appreciated.
If you have not, you might like to make use of the search feature before making a new ticket, as it's likely that the issue has been reported already.
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Seriously, nobody is doing anything about this?
Welp, that is majong's bug tracker in a nutshell.