Re: why there is no fast scrolling
Um because it was never implemented?
Re: adjust or disable large message view
Didn't get this part.
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This separate window is only used when message text is exceptionally large. Approx. close to two megabytes. Very uncommon.
I'd say that missing a fast scroll in large message view is a bug.
Adjust or disable large message view = add option to choose from what size message is considered as large or disable this considering at all and show all messages as normal ones.
BTW, Profimail had option in long message view - message end, message start.