Arduino Uno is mostly 1-clock per instruction. A 16MHz ATMega328p scores 16MHz MIPS.
80386 was mostly 4-clocks per instruction. A 20MHz 80386 only scored 5MHz MIPS. Most 1980s/90s computers were multiple-clock-ticks per instruction (8051 was 12-cycles per instruction, so a 12MHz 8051 is only 1 MIPS).
I don't think Intel got to single-cycle instructions until 80486?? Hard to remember all the differences...
Furthermore, the ATMega328p has many 16-bit instructions, like add, which operate over two registers in one clock tick.
It's 8bit, any operations with wider numbers require several instructions.