Any changes to firewall filter term or changes to Policers or prefix-list which are called under filter will refresh the respective filter causing a delay in SNMP Walk based on the size of the filter. 

As per firewall filter  test-filter, it has 1200 terms. The changes made on any term will refresh complete filter.

set policy-options prefix-list PL_8 10.10.10.0/24
set policy-options prefix-list PL_8 11.11.11.22/32
set firewall family inet filter test-filter term 8 from source-prefix-list PL_8
set firewall family inet filter test-filter term 8 then policer 30M
set firewall family inet filter test-filter term 8 then accept
 
root@jtac-lab> show snmp mib walk jnxFWCounterDisplayName              
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.49.3 = 30M-1
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.50.3 = 30M-2
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.51.3 = 30M-3
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.52.3 = 30M-4
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.53.3 = 30M-5
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.54.3 = 30M-6
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.55.3 = 30M-7
jnxFWCounterDisplayName.11.116.101.115.116.45.102.105.108.116.101.114.5.51.48.77.45.56.3 = 30M-8
…
.
 
root@jtac-lab> show snmp mib walk jnxFWCounterDisplayName | count   
Count: 1200 lines
 
Deleting the prefix from prefix-list which is called under term 8:
labroot@jtac-lab# show | compare
[edit policy-options prefix-list PL_8]
-    10.10.10.0/24;
 
root@jtac-lab# commit

root@jtac-lab> show snmp mib walk jnxFWCounterDisplayName | count   
Count: 200 lines
 
root@jtac-lab> show snmp mib walk jnxFWCounterDisplayName | count   
Count: 678 lines
 
root@jtac-lab> show snmp mib walk jnxFWCounterDisplayName | count   
Count: 1200 lines

This is an expected behavior of the firewall filter. The filter have to be re-programmed, which results in a delay in SNMP walk depending on the size of the filter. 

Here are the supporting logs where the firewall filter is ​re-read and re-applied. This can be validated from /var/log/messages:

Jun 17 10:11:40.825754 re-reading configuration, PID 9024
Jun 17 10:11:40.827123 mib2d_read_config: Done reading config...code:0
Jun 17 10:11:40.851572 fw_async_callback_gencfg: Incoming object with major type 15 minor type 21
Jun 17 10:11:40.851617 fw_async_callback_gencfg: Got filter name f1 with filter index 2, is_remote=0
Jun 17 10:11:40.851626 fw_async_callback_gencfg: filter: f1, filter index: 2, template_index 0, op: 2