We have a barberry too and you're right, it seems to be a very slow growing shrub. In two years, it's only grown one foot. The burgundy red foliage is very attractive though so we'll leave it where it is.
I wondered if it was because it’s surrounded by maybe too many plants but have just checked online and it is actually a slow grower. Also it’s due flowers in spring which I didn’t realise. It didn’t have flowers this year, so maybe next 🤗 @gjones