Spark 2.6.3 presents very slowly

Hello friends,
I do not have much to integrate this community and I have a problem I hope can help me.
at work we installed Spark 2.6.3 but several users have very slowly, take long to get the message, as well as they write and take to reach your destination, I could do in this case.

greetings and thanks.

2.6.3 and 2.7.x (543 and 545) very problematic versions, file transfer and presents. We use 2.5.8 (only one bug, chat screen no esc char to close).

So I’m thinking if I go back to version 2.5.8, which is more stable. thank you very much for the help.