Hi,
System performance depends on multiple things i.e. hardware (RAM, CPU), Network speed SAP Workprocess, system memory parameters and load on system at particular time.
From the above screenshots we can see all the dialog WP are utilised hence system will not allow any new sessions or login till any WP gets free. This could be the reason of deadly performance of your system.
Check the hardware capacity of your system, adjust the parameters and than increase dialog WP.
Another option is to install new DI server and use load balancing.