For the last few weeks Avast in really slowing down my computer every time I boot up
Which other security programs do you use?
and it appears to be constantly updating the programme or definitions
How did you set them to update? Automatically?
avast checks if there is an available connection each 40 seconds.
If there isn't, wait more 40 seconds to check. Checking does not take more than one second and, of course, does not use the Internet band.
If there is a connection, check for an update. If there is not any new file to download, wait 4 hours to start checking again. If there is an available update, start it and install it. Again, wait 4 hours to check the next time.
Some of this settings could be configurated different, I posted the default values of the avast4.ini file. The updates are incremental.
The program updates are released but to avoid excessive load on the servers the auto update check happens randomically every 7 days.
and it switches off the firewall in the process.
No, it does not. Are you sure you're not infected and the virus disabled the firewall?