The listed 3 points would more or less have affect of performance per my experience, however I've never experienced a perfomance as slow as 25-30 seconds. It usually doesn't take that long when loading a report.
One known fact that impacts the performance a lot is using a Direct Query to Azure SQL DB or on-premise database, the query would run for long if the data in DB is large.
By the way, you could try to debug the performance by using the brower dev tool(F12 in Chrome,IE,FF) to find what network traffic consume long time.