Also you may do a search for this topic, it is not so simple as higher MTU == better/faster, it also has some shortcomings. Larger packets may be more susceptible to noise => more errors/retries when 2.4GHz radio band is crowded. Also larger packets may not fit well into connection interval (typically there can be more smaller packets sent in same connection interval) so larger MTU may not increase speed and can even make things slower.
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Also you may do a search for this topic, it is not so simple as higher MTU == better/faster, it also has some shortcomings. Larger packets may be more susceptible to noise => more errors/retries when 2.4GHz radio band is crowded. Also larger packets may not fit well into connection interval (typically there can be more smaller packets sent in same connection interval) so larger MTU may not increase speed and can even make things slower.
EDIT: good article is https://interrupt.memfault.com/blog/ble-throughput-primer it gets more interesting near the end