I just wanted to know, if the dio packages uses isolates by default, if it do not uses isolates than how can I use them. because my API fetches a lot of data and decoding the json on UI thread might not be a good idea.
Dio has a plugin called dio_flutter_transformer that is especially for Flutter and will parse Json in a separate Isolate via the compute function.
Take a look at the Dio package on pub to see the list of plugins available: