I guess, I know why and when it happens.
This is what noticed during the block download. When my Bitcoin core downloads the blocks, the main interface that shows the Number of blocks left: gets frozen for a few minutes. When it starts updating the number of blocks again, the download speed decreases (sometimes it stops downloading the blocks). Now, when the Number of blocks left: gets frozen, we think it's not updating anymore. But this is the time when Bitcoin core downloads the blocks at full speed.
The possible reason for boosting the progress is, that when we restart the Bitcoin Core, It shuts down without updating the number of blocks left. But, once we start the Bitcoin core again, it synchronizes the downloaded blocks first and then starts downloading the next blocks and the number of blocks left gets frozen again or updated very slowly. I assume it happens due to the low RAM or CPU power.
DAY 4