Hi I've just taken up boxing I'm a bit overweight but when training my boxing coach says about puching faster is this something that comes with time or is it my excess weight slowing me down.??
Likely both, but moreso the time and practice part. Even an extremely lean and fit person is unlikely to be able to punch fast until they have repeatedly hit the bag for months, so don't be too hard on yourself, and don't expect results overnight. It takes most people months and tons of repetition to punch with speed and power. Welcome, and enjoy your training.
Probably slows you down ever so slightly, but nothing that couldn't be counter-acted by practicing throwing with no wasted movement (looping your shots around before throwing them, pulling the jab back then firing it instead of just firing it). I wouldn't worry about it though man, just keep working out, hitting the back, and doing your thing.. and you will get better and faster... and the weight will start to come off too.
Make sure you're relaxed, if you're too tense your punches will be stiff and slow. Just relax and snap.