Generally speaking, the faster the decision, the more likely it's an approval.
A refusal takes time to go through - they have to check every document twice, possibly make enquiries, maybe request further information from you... which can take a few extra days.
However, if the application is straightforward, you meet all the requirements and you've included every document, then the decision is easy - they just have to go down their checklist, tick off all the documents and issue the visa... only takes an hour or two.