Well Exchange 2019 is much more enhanced and a better version than Exchange 2016. I would like to summazire some of the performance improvement in Microsoft Exchange Server 2019 for you.
Improved search infrastructure: The completely rebuilt search infrastructure for cloud scale and reliability in Exchange Online is now available in Exchange 2019. This new search infrastructure allows for indexing of bigger files, simpler management, and better search performance.
Faster, more reliable failovers: The changes to the search architecture result in significantly faster and more reliable failover over between servers.
Metacache database: Improvements at the core of Exchange's database engine enable better overall performance and take advantage of the latest storage hardware, including larger disks and SSDs.
Modern hardware support: Exchange now supports up to 256 GB of memory and 48 CPU cores.
Dynamic database cache: The information store process employs dynamic memory cache allocation optimizing memory usage to active database usage.
I hope this helps you out. You can also contact me at firstname.lastname@example.org to learn more about Exchange Server 2019 .