what is the fastest way from Shanghai Pudong international airport to Hongqiao railway station.

Shuttle bus line 1 can take you to Hongqiao Railway Station directly.
In usual time, the duration is about an hour. If it is traffic jam, it will take 1.5 to 2 hours on the way.
Or you can take subway line 2 at the airport, get off at Hongqiao Railway Station, Exit E. The wholeway takes around 1.5 hours. It costs CNY 8 per person.

If taking taxi will it be more faster?

Yes, generally speaking, taking a taxi is the fastest way, about 50 minutes and CNY200 if there is no traffic jam. But when stuck in a traffic jam, subway would be the fastest.