How slow? LaserJet 4 are pretty old now. Heck, even my LaserJet 6 is kinda ancient.
As I recall though LaserJets were fairly slow until the 5/6 series. I got lucky, I went through a bag of SIMMs and managed to find a 16MB and two 8MB SIMMs that worked in the 6p which DRASTICALLY increased its speed.
However the combination of an Amiga with an LaserJet 4, well you can expect it to be slow. Depending on your Amiga the LaserJet might even have a faster CPU - LOL.