R-Variables Read/Write speed degrading over time Siemens 840D NCK V4.5|OPC UA Implementation: Stacks, Tools, and Samples|Forum|OPC Foundation

Hello and thank you in advance for reading. 

I am having an issue with a machine running Siemens 840D NCK V4.5, reading and writing R variables from OPC UC client, UAExpert. 

The problem is, that as time goes on in the sampling, the speed of the read/write of the variables slows down significantly over time. I am curious to know if there is any way to determine the exact cause of this, and to potentially improve the sampling rate. 

Thanks again for any advice.