speed确有速度、急速之意,英文解释如下
速度：===distance travelled per unit time
同义词：velocity
急速===a rate (usually rapid) at which something happens
同义词：swiftness,fastness
例句：
This plane can reach speeds in excess of 1000 kilometers an hour.
这架飞机可以达到每小时一千公里以上的速度.
The police are advising motorists to reduce speed because of the fog.
由于有雾,警察告诫驾车者减慢速度.
rapidity的意思是急速、快速之意
a rate that is rapid
同义词：celerity,quickness,rapidness,speediness
例句
We are surprised by the rapidity of her recovery.
她恢复得那么快,我们都有些惊讶.
