This is actually not a question but answer to the same topic closed some time ago. Maybe someone will find it useful.
So, Tmc2209 documentation says that VACTUAL is "motor velocity in +-(2^23)-1 [µsteps / t]"
But does not explain what is t.
Luckily, there is a document https://www.analog.com/media/en/engineering-tools/design-tools/TMC2209_Calculations.xlsx with some formulas.
t appears to be not time in seconds. For 12MHz Tmc2209 generator, which is default value, t = 2^24/12000000 = 1,398101 sec
Therefore, if you run in full step with VACTUAL=200 one full rotation will take not 1 second, but 1,398101 second