inputting data very slowly between 5 and 7 at night (every night!)

We are an organization who has a license on Parallels Client. All our online users uses Parallels Client.
We have the following problem:
Every night between 5 and 7, when we connect with the Parallels Client to our online bookkeeping program, we do notice that there is a mainly a problem with the input of data. (inputting the data is very slowly)
We have had everything examined by our IT department. They find nothing. No non-standard services, no excessive load,

Could the Parallels Client be the cause of this?

Do you have more tips? What can I examine in Parallels Client to maybe solve the problem?