network: slow Nmap scanning in some IP ranges

I am a security rookie trying to scan a private network in the range – using nmap on Kali Linux.

The routing table:

route -n

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface         UG    100    0        0 eth0   UG    1      0        0 tun0   UG    0      0        0 tun0   UG    1      0        0 tun0   U     100    0        0 eth0   UG    0      0        0 tun0   U     0      0        0 tun0

I tried to scan the entire range of the private network at once, but this caused nmap to close after a period of time.

Then I tried to scan blocks of 256 IP addresses at once, starting with

nmap -sS

Scans in the range – are completed quickly, but the scans in the range – are exponentially slower and generally do not complete.

Is there anything in the network routing that is affecting this? Is there something I am missing completely?