grammar - が早いか versus や否や

が早{はや}いか and や否{いな}や both seem to mean "as soon as". They describe a relation between two actions, where one action occurs when another does.

I'm a little fuzzy on the difference, though. I think, from reading examples in my N1 practise book that が早{はや}いか is more momentary. Action A is something that is more or less instantaneous, triggering B to happen in a mere moment.

や否{いな}や, on the other hand, can be a little more overlapping. While action A is in the process of completing, B happens.

Here are some example sentences which seem to support that difference:

時計{とけい}が10時{じ}を告{つ}げるが早いか、いっせいに問{と}い合{あ}わせの電話{でんわ}が鳴{な}り出{だ}した。

The clock hitting ten is an instantaneous occurrence.

母{はは}の足音{あしおと}が聞{き}こえるや否{いな}や、愛犬{いあけん}のチロは玄関{げんかん}までダッシュした。

Footsteps continue over a small amount of time, so there is a bit of overlap between them being heard by the dog, and it running toward the front door.

However, this example sentence doesn't fit the mould:

電車{でんしゃ}のドアが開{ひら}くや否{いな}や、どっと乗客{じょうかきゃく}が降{お}りてきた。

A train door opening, to me, seems more of an instantaneous occurrence than not. Though maybe that's just my take on it. I could see how some might argue that it is a point in time more than a section of time.

Am I correct in that they a differentiated by how instantaneous event A is? Is there anything else that differentiates them?

Answer

"Japanese Core Words and Phrases" by Kakuko Shoji has a little bit about ~が早いか. The book contrasts it with ~たと思ったら instead of with ~や否や, saying that "the focus here is on what actually happened rather than on what the speaker perceived". Unfortunately, the book doesn't have anything on ~や否や.

Looking at the sample sentences for ~や否や on this JLPT practice site, the examples aren't necessarily consistent with the idea that in Aや否やB, the two events can be overlapping in time. For instance, the two events in the below example should be instantaneous:

その部屋に入るやいなや彼らは話をやめた。
The minute I entered the room, they stopped talking.

The description on this grammar site suggests the two are essentially the same.

This Chiebukuro answer suggests the same thing, but adds that ~や否や has the additional meaning ~かどうか, and clearly cannot in this capacity be substituted with ~が早いか.