webRequest is slower because it has to evaluate JavaScript for each request (as well as the overhead of interprocess communication), instead of the blocking being done by compiled C++ code in the same process like declarativeNetRequest does.

uBO also has a bunch of extra features like zapping that the creator explicitly chose not to include in uBO Lite, in the interests of making the Lite version as fast and resource-light as possible. For zapping, there are other extensions you can install instead if you need that.

They're two different products with two different philosophies based on two different underlying architectures. The older architecture has now gone away in Chrome, but the new one supports uBlock Origin Lite great.