Not usually. Hydrocele which is fluid around the testicle does not usually cause symptoms unless it arises suddenly which may cause discomfort from stretching of the scrotum.
Not necessariiy. Most hydroceles are painless. They may look painful duto their size. They come in two flavors: communicating and noncommunicating. A communicating hydrocele changes in size as fluid goes to and fro betwenn abdominal compartment and the scrotum. A noncomunicating hydrocele is static in size and may resolve if it occurs at birth. Surgery will fix communicating hydrocele.
Usually slowly... Usually, it comes slowly especially in idiopathic case which may take yrs or even decades to fully developed. But it is infection-/tumor-related, it may be just days to weeks to develop. But analyzing hx can discern their difference. So, it's usually painless, but its growing bulkiness may induce some mental anguish and physical inconvenience.