Assistance with CRM built on MS Access and running on Win Server 2016 via MS SQL

Good day. 

I would like to inquire if someone can help us understand the operation of the CRM program "Client Accounting"? 

The program is built on MS Access, we work with it through Windows Server 2016 locally over the network or by connecting remotely via RDP. The database operates using MS SQL. 

Since I am not a specialist in this matter, I apologize if I may name or describe something incorrectly ))

The essence of the problem is... Once our CRM worked on server 2008 and 2012, probably the SQL itself was of a later version. After server 2008 stopped being supported, we switched to 2012 - everything worked quickly on both versions. But last year, due to a server breach, we restored everything and decided to switch to Server 2016 right away, and after that, the program started to lag terribly when working in it via RDP and noticeably lag even during local work in the office just over the network. 

The thing is, the developer is Russian, and now we do not want to resolve this issue with their technical support. 

And it is unclear what the cause is here, maybe the server (or some network restrictions, or maybe security parameters, etc.), maybe the MS SQL DBMS, maybe somehow the program itself (but I think this is less likely)... 

Perhaps it is necessary to monitor some parameters of the DBMS and see what time is being spent and where these delays are occurring. 

We tried to reduce the database by deleting information by 50-70% - this did not affect the speed of operation at all. 

Our specialists, represented by a system administrator-programmer and an acquaintance in servers, unfortunately cannot overcome this.

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