You are getting frame loss (RX-DRP) rates of between 0.3% and 1.6% on your interfaces due to buffering misses which is probably the main thing slowing you down. This is almost certainly due to high CPU load on your 2 cores, given the large number blades you have enabled on an old 2-core box like that 4200, 100Mbps top throughput doesn't seem that unreasonable to me. Currently you have a 2/2 CoreXL split on your box, in some cases disabling CoreXL and going to a 1/1 split helps on a 2-core box but given your high PXL% I don't think doing that will help in this case.
The 4200 only has 4GB of RAM which may not be enough for all you are trying to do. Please provide output of the free -m command to see if a memory upgrade will help.
You can probably pick up some more speed by tuning your policies, the two major areas in your case are Threat Prevention and APCL/URLF. In order to figure out where to focus your efforts, try this and report back what you see:
1) Run Internet speed test and note throughput
2) On the gateway from expert mode run commands ips off and fw amw unload
3) Wait 60 seconds
4) From a completely new browser instance run an Internet speed test and note throughput. If throughput has substantially increased you need to tune your IPS & Threat Prevention configuration.
5) Run commands ips on and fw amw fetch local
6) Wait 60 seconds
7) From a completely new browser instance run an Internet speed test and note throughput. (should be about the same as #1)
😎 On gateway object in SmartConsole, uncheck the APCL and URLF blades and reinstall policy to the gateway.
9) Wait 60 seconds
9) From a completely new browser instance run an Internet speed test and note throughput. If throughput has substantially increased you need to tune your APCL/URLF policy, typically this will involve removing the "Any Any Any Accept" rule at the bottom of your APCL/URLF policy (which is not necessary except for logging purposes), and making sure you are using object "Internet" in the Destination column of all APCL/URLF rules and NOT "Any".
10) Recheck the APCL and URLF checkboxes and reinstall policy to the gateway.
11) From a completely new browser instance run an Internet speed test and note throughput. (should be about the same as #1)
Let us know what you find out.
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