Why will writing to force merged indices make performance "much worse"?!

I have been following the changes to the documentation regarding force merge warning. I understand that force merging to an index that's actively being written to is a bad idea. However, if I use force merge to bring down the number of segments to n, wait for it to complete and then write to the index, why would it make the performance "much worse" according to the warning? I just have to make sure that max segments to merge to isn't too few so that >5GB segments are created, right?
Thank you

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