I tried to use the netboot feature of the RPi3 and booted via network. Therefore I used dnsmasq of pihole, tftpd-hpa and NFS on another Rpi3. I want to use the RPi headless. The Boot takes up to 30minutes and the ssh logon took more than 5 minutes. I checked the average loads and they had values greater than 10 but the cpu-usage is lower than 20%. The original image is much faster and there is no lack. How to analyze the problem better? I can’t imagine that’s normal.
What model exactly do you use from the RPi3 family? Only model 3B+ is optimized for PXE boot. It may be possible also for model 3A+ because it is a stripped down 3B+ but I don't know. So if you use model 3B it may be one reason for your problems.
Then with dnsmasq, pihole, tftpd-hpa and NFS there are some sophisticated applications that must be made to play together. In particular pihole and tftpd-hpa are not needed and complicate things unnecessary. A tftp server is already provided by dnsmasq.
How to setup a proper booting PXE only with dnsmasq and nfs-kernel-server you can find at Netbooting multiple "workers" RPi from a "master" RPi. You may give it at try.