Having fast C++ code is always good even if its not directly responsible for execution, but yes typically order entry and trade parsing at really serious HFT firms is done with an FPGA/ASIC. Sometimes a trade packet wont even hit the software and instead be processed entirely on the FPGA.

Some neat tricks people have done is to actually process packet bytes in order as they come in on the FPGA, and if it has some actionable header they just send an action before the entire trade packet is received. With that you can go down into the few ns for your w2w latency.

When I left, they were experimenting with an FPGA implementation of the TCP stack (!). Obviously a very challenging undertaking, not sure how far they got.

Is C++ slow even if you don't use any language features that add runtime overhead, e.g. runtime exceptions?