Navigation Key presses randomly swallowed

For quite a long time now, I’ve been finding that navigation by key is very unreliable.

Specifically trying to select a bunch of messages by the keyboard, using shift + down arrow results in about a 50% drop rate of the key presses.

What I mean is if I have 6 messages to select, it should be hold shift + 5 x down arrow, but I usually find it takes about 8 - 10 to do it.  Even without shift (not selecting) it drops / discards the key presses, and I have to be extremely careful when navigating.

It appears to be related to the display of preview messages in the preview window.

If I navigate quickly, it doesn’t  try to display the preview, and it doesn’t drop messages.

If I go slowly and wait until the preview is shown before hitting the key again, it’s also fine (but very slow, IMAP).

If I try to use a normal speed, it drops the keypress if it’s currently rendering a preview.

This is not a good design choice for the reasons above.  People should be able to rely on the number of times they press a key being the number of times the program acts on it.

I wonder if you turn off conversations (Menu > View > Conversations) if there will be any difference.