At 16 MHz, the STM8 pipeline delivers roughly 16 MIPS. That is enough to run a PID loop at a few kHz, poll a UART, or toggle a GPIO at the bus rate.
Memory sizing: 8 KB Flash, 1 KB RAM, 640 B EEPROM
The 8 KB Flash (8K x 8) holds firmware for a single-purpose application — a Modbus RTU slave, a PWM fan controller, or a keypad scanner. The 1 KB RAM is enough for a few dozen variables and a small stack. The 640-byte EEPROM stores calibration constants or configuration parameters that survive a power cycle. Plan your memory map.
The 20-TSSOP package is surface-mount, so plan for reflow soldering and a conformal coat if condensation is expected.