delayed Waveform.onfinish preventing a new startInput?

I'm using Waveform to rapidly read a sequence of data, very successfully. The code is like this:

wfrm = new Waveform(5)
wfrm.on('finish', callback)
setInterval(() => {
  wfrm.startInput(A0, 50)
}, 500)

function callback(buf) {
  // do something with buf
}

Every 500 ms a 'task' is started to do an A/D converesion, 5 times, at 50 Hz.
When ready the callback evaluates the passed Uint8Array (argument 'buf') very quickly -> no problem at all.
Sometimes, however, there are errors

Uncaught Error: Waveform is already running
Uncaught InternalError: Timeout on ADC

Sometimes the callback has to do more, including an HTTP call. This might take longer than the repat interval of 500 ms an cause these errors.
Does a long running callback (longer than 500 ms) really make the Waveform complain at the next startInput, as I'm thinking?
Is there a way to effectively make the callback appear as completed although parts of the code still need to be executed - like process.nextTick?