In our loopback lab, a UTCP call took a median 0.161 ms, plain HTTP 0.162 ms and MCP 0.632 ms.
Tool definitions were 6,783 bytes for UTCP and 6,052 bytes for MCP across 10 tools, so neither saves context.
Pick MCP when third-party AI clients must call your tools, and UTCP when you control the agent and already have HTTP APIs.