thanks!
I found this reference " The capacitive load when transferring a signal off-chip to the surrounding world is about 10-50 pF, which indicates that we need a transistor 1000 times stronger to drive the off-chip load. Therefore, output buffers are necessary when the circuit is sending information out from the chip." in
https://www.isy.liu.se/edu/kurs/TSEK06/kursmaterial/TSEK06_ProjectGuide_1.5.pdf
I remember I was using around 10pF some time ago, I wanted to confirm with other