If you really want to uncompress them in parallel, you could do

for i in *zip; do unzip "$i" & done

That however, will launch N processes for N .zip files and could be very heavy on your system. For a more controlled approach, launching only 10 parallel processes at a time, try this:

find . -name '*.zip' -print0 | xargs -0 -I {} -P 10 unzip {}

To control the number of parallel processes launched, change -P to whatever you want. If you don't want recurse into subdirectories, do this instead:

find . -maxdepth 1 -name '*.zip' -print0 | xargs -0 -I {} -P 10 unzip {}

Alternatively, you can install GNU parallel as suggested by @OleTange in the comments and run

parallel unzip ::: *zip