Heroes of Might and Magic V: Tribes of the East (Steam license) is lagging.

0) Update the drivers for your motherboard, sound card, and video card!
1) If that doesn't help, you need to install DirectX 9.0c, but it can't even be installed on Windows 10! What should I do?

1) Run appwiz.cpl (or open Programs and Features in the Control Panel)

2) In the left column, select the third item: Turn Windows features on or off

3) Find and check the box next to Legacy Components and the submenu DirectPlay

And here's a link toDirectX End-User Runtimes (June 2010) - https://www.microsoft.com/en-us/download/details.aspx?id=8109
The file directx_Jun2010_redist.exe will be downloaded. - You need to place it in a folder and run it, as it is a self-extracting archive, so you need to specify the extraction folder; it is best to specify the same folder as the default! After extraction, you need to run the file DXSETUP.exe as an Administrator!
Reboot the OS...