I have never really had slow boots even on this old XP Pro SP3 system and I have been using avast for over eleven years on various systems. This particular system even though old (about 6 years) still has a reasonable specification (CPU & RAM) and avast hasn’t really impacted that.

What we don’t know is anything about your system specification and any other security based software ?

You might also want to check those programs that when installed want to run on boot, even if they aren’t required that early. Typical of those types of program are media players, they only need to run when you want to play media files.