I have put both observations from April 15 and April 22 together
in the attached images-set.Obvious was is the very different
longitudes what suggest a great drift in one week time.

Here is a crude analysis of these observations from 
NASA Cassini-specialist Georg Fisher;

I think you do observe the same spot, and the drift speed is not abnormally high. Just a quick and rough calculation:
I estimate this spot in the STeZ to be at a latitude of about 50°, where it is known from the Voyager observations 
that the wind speed can be even higher than 100 m/s. The circumference of Saturn at 50° is about 
2*pi*60000*cos(50)=242 000 km (not taking the oblateness into account), which corresponds to 360° in longitude, i.e. 
1° in longitude corresponds to about 670 km at 50° latitude (it is about 1000 km at Saturn's equator). A speed of 
100 m/s corresponds to 8640 km/day, which would be 8640/670=13° per day. So in one week (or 7 days) the longitude 
should change by about 7*13=90°, and I think that is approximately what you observe. The drift is eastward, so the 
western longitude of the spot seems to change from about 160° at April 15 to 70° at April 22 (estimated roughly from your images).
 So within one week some features can considerably change their longitude, and this effect is greatest at the equator 
where we have wind speeds up to 400 m/s, but it is marginal at 35° South, where we have observed our prominent white clouds.

[バンディバ:オランダ]
[Ralf Vandebergh:Neighbourhood of Maastricht Netherlands]