If you are fully synchronized but your block height is not the same as other people, you are not fully synchronized or are connected to bad peers. Check against know block explorers (e.g. xmrchain.net) or remote node operators (like xmr.to).
As repeatedly pointed out, churning in most cases is actually harmful to your privacy. The only exception I'm aware ...
Possible causes explained to me in #monero:matrix.org:
Verification tends to be limiting factor.
Using an HDD is known to be significantly slower than SSD/NVMe.
Apparently HDD seek time is a significant bottleneck.
Verification operations are not very parallelizable therefore if a single CPU core is spiked to 100%, that could be limiting factor, even if ...