“I have gained 30 pounds since my last 10k or 5k, how does extra weight effect race performance?”
If the gained weight is only fat, then the most obvious effect that weight gain will have on performance is the impending slow down in pace due to the effort it takes to move the extra weight down the road. In general, for extra fat weight, then one can expect to slow down about a second per mile per pound. So for every 10 pounds in fat someone is carrying, they will run 10 seconds or so a mile slower than before given the same effort. The extra body fat also throws the body’s hydration and cooling mechanisms out of wack, so one must take that into consideration as well.
However, if a runner has added some functional muscle tissue, then that won’t be the case. For some individuals, adding 5 lbs of muscle helps their running. They run faster, have healthier immune systems and recover faster. It is recommended, for those not racing at very elite levels, to push for maintaining functional muscle tissue as part of their year round fitness routine. Not that elites don’t need muscle tissue, but they do have to whittle down to the bare minimum. Everyone else can and should carry more.