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XDATA vs IDATA System Overhead with Modern 8051s?

I'm wrapping up some product code on a modern 8051.

I actually surprised myself, because I ran out of memory in the data area.

I need to check with the manufacturer because supposedly this chip has 256 bytes of RAM in both the IRAM and XDATA. I have to dig through the device setup files from the manufacturer, because it isn't like an ARM where you specify the address ranges in KEIL directly.

I understand legacy 8051s had an external memory source in some cases, that you could access using the XDATA syntax. I understand the architecture issue to a degree since I've been building a mock 8 bit MCU in Verilog.

On a contemporary 8051, is it really that big of a performance to use XData area for variables?

Am I correct this is really a micro-optimization in terms of system gain? Like fractions of microseconds (µs) difference, or is it worse?