You're question is very poorly laid out, but I'm going to assume the following:
Users with Quickbooks are complaining that it's slow to load/save/etc.
Step 1: Make sure all machines are connected to the LAN via Gigabit Ethernet and not WiFi. WiFi is great and all and it would probably work, but it's not ideal, especially for an office environment with potentially heavy load
Step 2: Make sure the Server hosting the SMB and Quickbooks file is also on Gigabit Ethernet.
Step 3: Make sure all of the local machines aren't 15 years old and have recently new hardware; ideally SSD's. They should also have at least 16GB of RAM (if not more) to be able to handle the load of the OS and the Quickbooks application and the large file they have to open for modification.
Step 4: The only way to speed up the delivery of such a file from the server, would be to ensure that the disk speed is fast enough to access the file and serve it via the NIC at the highest possible throughput. Make sure the disks are fast (if not using SSD), possibly in a RAID array (which is NOT a backup) and that the server isn't also under heavy load from other tasks.
Step 5: Make sure there's a good amount of RAM on the server to be able to handle the multiple open sessions.
Step 6: Make sure the server is local to the users. Aka, it's not over some site-to-site VPN or cloud-hosted thing.
Assuming Step 1, 2 and 3, the bottleneck will likely be how fast the file can be read into memory for SMB to deliver to the clients (Step 3). You can only do so much to make this as fast as possible before the costs outweigh the gain.
As a side-note, it doesn't sound like this "server" is really a server, and just more a less a workstation with the role of a server. You may need to invest in actual hardware designed for this task. If you're running Windows with 8GB hosting this file to 15 clients at 500MB a pop, you're already at a loss. If this is Windows Server with 8GB, you really need to bump that up. If you're going to run a server, it should also not be an i5 for production services; at least an i7, if not Xeon class. This setup is fine for small office needs, but it seems that you have exceed the capabilities of what it can handle.
Working directly off of an SMB share (or any network share) is rarely considered a viable working solution. Typically the recommendations are to make a local copy, do the edits, and then copy that back to the share.
Now, with all of that said, contact Quickbooks and see what solutions they offer for this. I highly doubt they designed the application to work in this method (maybe? maybe not? I'm not a Quickbooks user/administrator) but they very well may have a product that's more suited to this type of usage that doesn't require that the user load the file across the network every time they need to make a change.