が早いか|がはやいか

1. 1The moment A happens, B follows immediately, with almost no interval between them.Ex. 1
2. 2Right after someone does or says A, B takes place at once.Ex. 2

Used mainly in written narrative, novels, and descriptive accounts to stress the speed of a reaction or a follow-up event. The second clause usually describes a completed, factual event, often in the past tense. Compared with the conversational たとたん, it sounds literary. It is close in meaning and register to や否や.

• や否やN1「や否や」 means almost the same as 「が早いか」, and the two are often interchangeable when describing other people or things. However, 「や否や」 attaches only to the dictionary form, can be shortened to 「〜や」, and can also be used in first-person narration, while 「が早いか」 puts stronger emphasis on speed that almost preempts the first action.
• なりN1「なり」 attaches only to the dictionary form of verbs, never the た form; its second clause is typically an unexpected action, the subject is usually the same in both clauses, and the second clause can take 「のだろう」 to express conjecture. When narrating something that has already happened, the two are often interchangeable.
• そばからN1「そばから」 stresses that the same thing keeps happening and the effort of the first clause goes to waste, often with a nuance of resignation or dissatisfaction; it cannot be used for one-time events and works in both writing and speech. 「が早いか」, by contrast, centers on a surprisingly fast transition and belongs to written style.
• たとたんN3「たとたん」 also means 'as soon as one thing happens, another follows,' but it is register-neutral and common in speech, making it the better choice for everyday conversation; it attaches only to the た form of verbs. 「が早いか」 leans toward written style and more strongly conveys speed that almost preempts the first action.
• た拍子にN1「た拍子に」 stresses that the impact or momentum of the first clause's momentary action unexpectedly triggers the second clause, which is usually a non-volitional, unexpected outcome and often takes 「てしまう」; it attaches to the た form of verbs or 「名詞＋の」, and is register-neutral. It cannot be substituted when you simply want to say that one event follows another in time.
• かと思ったら / かと思うとN2「かと思ったら / かと思うと」 also means 'as soon as one thing happens, another follows,' but it attaches to the た form of verbs and highlights the contrast and surprise between the two clauses rather than speed; it is register-neutral and common in speech as well. 「かと思ったら」 additionally carries the meaning 'I thought..., but in fact...'.

や否や is nearly synonymous and equally literary, and なり stresses one person's immediate follow-up action. たとたん is more conversational and often implies surprise. た拍子に marks an accidental result triggered by A, and かと思ったら/かと思うと highlights the speaker's surprised perception. そばから describes something that is repeatedly undone right after it is done.