USPS Parcel Select ground shipping is not slow unless you compare it to USPS priority mail.
USPS Parcel Select Ground is a standard shipping service that delivers your mail within 2 to 4 days of the package being shipped out.
Priority mail is faster than USPS Parcel Select ground select service and takes around 1 to 2 days while USPS Parcel Select ground select usually takes a few additional days.
If you have something coming to you through USPS Parcel Select ground service then you should get the package within a few days.
The USPS Parcel Select ground is just a standard package shipping service through USPS and takes around the same time as UPS shipping and FedEx shipping.
It's just a cheaper alternative to other shipping services.
Every time I've had a package shipped to me through USPS Parcel Select ground I got the package within 3 days.