No avast user can expand/respond to what you said as it isn't a specific question.
Do you really expect/think that avast aren't always striving to optimize the scanning engine as they have for many years. Adding more protection functions doesn't come without some system overhead. I have been using avast free for almost 13 years and it has changed massively in that time.
You don't even say what your system is and how much/what resources you have. I have three systems ranging from:
1. Win10 64bit laptop with Intel Core i5-7200U @2.50GHz to 3.1GHz, 8GB DDR4 RAM, 256GB SSD primary drive
2. WinXP Pro SP3 (see signature).
3. Win7 32bit, with a crap 1.5 Intel atom dual core CPU and 2GB of ram, even this is able to run the latest avast version. I always do a custom scan and only install those components that I actually need.