is there a performance hit? -> Intel i7 late 2019 MacBook Pro laptop using 38" ultrawide monitor

Check the 2nd display specs for your specific 2019 MBP model. There were 12 different 2019 models including 4 different i7's and some had different capabilities w/r/t 2nd displays. A good place to check is everymac.com. The 2019 MBPs have 4 TB3(USBC) ports, so your selected display will at least need to support TB3 connection (or DisplayPort if you use a TB3-DisplayPort adapter).

Then check the specs for the displays you are considering. They need to match what the MBP can support.

There should not be any performance hit.

the speed at which data must be fetched and sent to a display that large is so fast, it EXCEEDs the processor's ability to send data byte-by-byte, even if it were doing NOTHING else.

so your Mac does not try to do that. Screen re-painting is done by an automaton that fetches successive bytes from memory directly, and the processor is only involved once per screenful.

The data rates are faster and more unrelenting than any other data movement inside your computer except memory access. But with this Hardware Assist, of a dedicated rasterizer/display-generator, the additional processor load of connecting an additional large screen is negligible, provided your Mac supports a 5K display.