The tribulation and suffering that would last for ten days was most likely the times of persecution under the ten Roman Caesars since the birth of the new covenant Church.

Correction: a "day" in biblical prophecy equates to a literal year (Numbers 14:34; Ezekiel 4:4-6), meaning ten years of persecution were prophesied.

The last of them, Diocletian, had the most believers slaughtered.

Indeed, and the Diocletian Persecution lasted for ten years, from AD 303 to 313.

https://en.wikipedia.org/wiki/Diocletianic_Persecution