First place i will ask, is there any rules or changes to the FW compare to last good known config.
Other side check the port-negotiation and any errors. if possible change Ethernet cables both the sides to eliminate any hardware issue.
Since its not major network FW, if this home based as per my understand, Clear all the config, make it simple config and test it..if that is working then we need to look the rule base.
Do you have IPS modules or policies enabled ? Check the MTU ?
look the troubleshoot guide for refernce :
https://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-firewalls/113393-asa-troubleshoot-throughput-00.html