I have personally made several withdrawals from Just Markets and their withdrawals time is instant (around 1 minute) when you select the online bank transfer method.
If there is any network downtime that is going to affect withdrawal speed, just markets will send you a notification in advance (check your personal area) telling you that they are having network issues that will affect withdrawals.
I have not withdrawn using other methods apart from online banking, so I cant testify to their withdrawal speed.
