You mention that CPU power seems to be the bottle neck in your system. To understand whether this is caused by TWS or by Excel, could you stop the Excel program and see by how much the CPU load drops? This would show you how much CPU load is caused by TWS and drawing the charts on your monitors. This could also help in deciding whether you indeed need to change the maximum RAM heap size assigned to TWS.
Agree with this guy. Compare the Thread Count. TWS vs TWS + Excel. My guess is the data latency in IBKR feeds, not internet service, is causing Excel to wait to do recalculations. This results in additional thread management tasks. Bad spreadsheet design can do this also , but I will go on faith that you know what your doing. Your PC hardware is definitely overkill. I would game with that.
RTX 2060 has 1,920 CUDA cores and can do real time ray tracing. It is a little super computer. With a i7-9700 that is really nice. The problem is running really old software on a supercomputer. IB DDE into excel is just never going to be good. I love excel for small quick data problems. Using it for trading though is just never going to be satisfactory.