Ok, so here are my first 2 cents on this matter.
Setup:
CPU: Intel E7400
MB: Gigabyte EP31-DS3L
RAM: 4GB
HDD: 1 x 640 GB with 3 partitions (C: drive is only 80GB), 1 x 250GB for Games and Virtual machines I play with, 1 x 80GB for downloading
Video: Gigabyte Geforce 260 GTX core 216
OS: Windows 7 HP 64bit
Firewall: Windows has a capable firewall so I use it
Browser: IE8 with SmartScreen (Chrome and Opera)
On demand: MBAM and Hitman Pro
Daily C: drive snapshots with windows backup
Weekly C: drive snapshots with Macrium Reflect Free
I'm also behind a router that knows NAT and can do port forwarding and DMZ
I have freshly installed avast 5 after about 5 months of using no av and then MSE. I have disabled IM, Web, P2P and Net shields. Modified setting for both Quick Scan and Full Scan so they check against and populate the persistent cache whenever they run and ran a full system scan.
Today, reading a bit on the forum about web shields I did a small experiment. I enabled web shield and went to a few sites:
http://www.avast.com/1st time it failed to completely load, did a refresh and it loaded fine.
http://wow-europe.com/Loaded completely 1st time though a bit slow. Refresh was fast.
http://personalfirewall.comodo.com/free-download.htmlLoaded ok. Downloaded the install file for comodo firewall. got 1,5MB speed with a 2MB peak.
I then disabled webshield and redownloaded the file. I got a steady 3.2MB download speed.
Did another thing. Deleted cache with CCleaner and then loaded the above again.
http://www.avast.com/Loaded fine though a tad slow.
http://wow-europe.com/Loaded completely though a bit slow. Refresh was fast.
http://personalfirewall.comodo.com/free-download.htmlLoaded ok. Downloaded the install file for comodo firewall. got 1,5MB speed.
So my conclusion is that web shield will slow down browsing as long as at least part of th content is not already in system cache. During this small testing part avast never spiked my CPU, just choked web browsing a bit. But as I said, websites that you visit often should not be too affected. Just download speed for new content.