network: slow Nmap scanning in some IP ranges

I am a security rookie trying to scan a private network in the range 10.0.0.0 – 10.0.15.254 using nmap on Kali Linux.

The routing table:

route -n

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.0.2.2        0.0.0.0         UG    100    0        0 eth0
10.0.0.0        192.168.0.1     255.255.255.0   UG    1      0        0 tun0
10.0.0.0        192.168.0.1     255.255.252.0   UG    0      0        0 tun0
10.0.2.0        192.168.0.1     255.255.255.0   UG    1      0        0 tun0
10.0.2.0        0.0.0.0         255.255.255.0   U     100    0        0 eth0
10.0.4.0        192.168.0.1     255.255.252.0   UG    0      0        0 tun0
192.168.0.0     0.0.0.0         255.255.255.0   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 10.0.0.0/24

Scans in the range 10.0.0.0 – 10.0.4.255 are completed quickly, but the scans in the range 10.0.5.0 – 10.0.15.254 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?