slapcat slapadd
Outside of that, there's too little information here to go on. For example, same filesystem and same options for the filesystem? If it's cloud now, and was onprem before, what are the IOP restrictions, etc.
I am using slapadd. If I install from binaries on alpine (3.19) I have this slow writes issue also. How are you testing? I hope not on ssd/nvme.