The financial engine of the market for office buildings, hotels and malls is showing signs of strain, raising questions about the resilience of the commercial real-estate boom.

Bonds backed by commercial-real-estate loans have weakened significantly since the start of the year amid concerns of an economic slowdown. Risk premiums on some slices of commercial-mortgage-backed securities have jumped 2.75 percentage points since Jan. 1, a move that translates into a roughly 18% drop in prices for triple-B-rated bonds, according...