Cisco VPN :: ASA5510 - Slow Traffic On IPSec Tunnels

We have many VPN tunnels back to our corporate office.  All of these tunnels are very slow (same with our client VPN's).  Our main firewall device at the corporate office is an ASA5510.  We have a 100 Mb/sec Metro Ethernet internet connection here.  We do not allow split-tunneling.

Our remote sites vary.  We have DSL connections, cable internet connections, and other types of broadband that vary in speeds from 5 to 100 Mb/sec (up and down).  The remote sites mostly have PIX 501's, but we have an ASA 5505 in one of the locations.

To take an example.  On one of our remote sites that has a 100 Mb/sec connection, if I ping device to device, I'm getting ping times of about 50ms.  And I'm pinging back through another 100 Mb/sec connection.  If I get on a computer down there and run a speed test, I'm showing down speeds of about 1.5 Mb/sec... nowhere near 100.  Some of that could be due to the lack of split tunneling, but I also suspect this could be an MTU issue. 

Right now, all my MTU's are just set to the default 1500.  Perhaps this is too high.  I used this site to check my max: [URL]
 
I did a few tests from behind several of my firewalls.  I pinged from a machine on one side of the tunnel to the firewall on the other end.  I'm assuming the max MTU I come up with is the max MTU for the firewall I'm behind while pinging, right?  The max amounts I came up with for some of my devices were as follows: Corporate ASA 5510 > 1272 (if you add the 28 byte packet header that would make it 1300) Remote PIX 501 > 1416 (if you add the 28 byte packet header that would make it 1444) Remote ASA 5505 > 1418 (if you add the 28 byte packet header that would make it 1446)

So, do I just need to set my MTU values to the appropriate amounts?  I have tried changing the value, but I don't see any change in speed/performance.  But I also don't know if I need to reboot the firewalls after changing the MTU.  I know with Catalyst switches, you have to reload.  But I didn't see any messages about needing to reboot on the ASA's/PIX's.