There are several reasons why QuickBooks runs slowly in multi-user mode:

The data file size is too large to be handled efficiently by the system. (The size will vary from system to system.)
There are three or more users accessing one module (for example, Accounts Receivable or Accounts Payable), at the same time.
A large report is being run (for example, Transaction report) in data files with more than 32,000 targets.
The data file is located on a slow computer.
A Network Interface Card (NIC) is slow or defective.
To resolve this issue, follow the solution for your situation:

Limit the number of users accessing a module at any one time.
Narrow the date range on a large report.
Store the data file on the fastest machine on the network.
Replace the Network Interface Card.
Note: For additional information, follow these steps:

From the QuickBooks Help menu, choose Search.
Type performance in the Search box, and then click green arrow.
Select Improve performance in multi-user mode.

Thank you.