How to check my Pokémon's attack speed?

Pokemon Speed does not exist in Pokemon Go. The only stats are Stamina (the equivalent of HP from the main games), Attack (combined physical and special Attack) and Defense (combined physical and special defence).

Variations in the speeds in the attack of the Pokemon are due to differing speeds with the moves they used, not the stats. Pokemon using different moves will have different speeds. The attack speeds cannot be checked in Pokemon Go, but thankfully they have been datamined and can be viewed online. For example here is a list of fast moves and charge moves with their speeds.