Hi Josie (or whatever your real name is) and welcome to RIBnet.
We've been a couple of times from Southampton to Poole and it is a very pleasant journey on a nice day. It's about 40 nautical miles and took us about an hour and a half or so, cruising at 30 knots. The Needles channel can be a bit bumpy and Poole Bay can be very uncomfortable on a windy day. If there's an offshore wind, you could hug the coast but do get familiar with the chart so you know where the hazards are (especially Hengistbury Head and the shallow inshore approach to Poole).
Hope to see you on the water soon!